Optical semiconductor device

ABSTRACT

Provided is an optical semiconductor device including a laminate structural body 20 in which an n-type compound semiconductor layer 21, an active layer 23, and a p-type compound semiconductor layer 22 are laminated in this order. The active layer 23 includes a multiquantum well structure including a tunnel barrier layer 33, and a compositional variation of a well layer 312 adjacent to the p-type compound semiconductor layer 22 is greater than a compositional variation of another well layer 311. Band gap energy of the well layer 312 adjacent to the p-type compound semiconductor layer 22 is smaller than band gap energy of the other well layer 311. A thickness of the well layer 312 adjacent to the p-type compound semiconductor layer 22 is greater than a thickness of the other well layer 311.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 15/535,142, filed Jun. 12, 2017, which is aNational Stage Entry of PCT/JP2015/078087, filed Oct. 2, 2015, andclaims the benefit of priority from prior Japanese Patent Application JP2014-264732, filed Dec. 26, 2014, the entire content of which is herebyincorporated by reference.

TECHNICAL FIELD

The present disclosure relates to an optical semiconductor device.

BACKGROUND ART

Increase of the emission efficiency and reduction of the thresholdcurrent in various light emitting diodes or semiconductor laser devicesare essential for high output and improvement of power consumption orthe like and are currently being intensively investigated. However, in anitride compound semiconductor light emitting device which emits blue orgreen light, the amount of current injected increases as the emissionwavelength increases, which causes problems such as a reduction in theemission efficiency and an increase in the threshold current. One causeof these problems is non-uniformity of carriers in an active layer(i.e., a light emitting layer). That is, the energy gap differencebetween barrier and well layers which constitute a multiquantum wellstructure increases as the emission wavelength increases. In addition,when an active layer is formed on a c-surface of a GaN substrate,piezoelectric field effects occur in well or barrier layers such that itis difficult for carriers (electrons or holes) to exit a well layer oncethey have entered the well layer, thus causing non-uniformity ofcarriers in the active layer (light emitting layer).

An example in which such a phenomenon is represented by numericalcalculation is described in Non-Patent Literature 1, IEEE, Journal ofSelected Topics in Quantum Electronics Vol. 15 No. 5 (2011) p. 1390.According to this Non-Patent Literature 1, the difficulty for carriersin a well layer to exit the well layer when the emission wavelength isequal to or greater than 400 nm in the case where an active layer isformed on a c-surface of a GaN substrate, and when the emissionwavelength is equal to or greater than 450 nm in the case where anactive layer is formed on a non-polar surface of a GaN substrate, isillustrated by a relationship between an emission recombination time andthe time required for a carrier to escape from a well layer (see FIG.12). In FIG. 12, “A” represents the behavior of a hole in the case wherean active layer is formed on a c-surface of a GaN substrate, “B”represents the behavior of an electron in the case where an active layeris formed on a c-surface of a GaN substrate, “a” represents the behaviorof a hole in the case where an active layer is formed on a non-polarsurface of a GaN substrate, and “b” represents the behavior of anelectron in the case where an active layer is formed on a non-polarsurface of a GaN substrate. Typically, carriers move between well layersin a multiquantum well structure having two or more well layers in avery short time of about 100 femtoseconds or less. However, for theabove reasons, the time required for a carrier to escape from a welllayer is increased and electrons or holes cannot freely move betweenwell layers. As a result, the electron and hole concentrations in eachwell layer differ from each other and remaining carriers do notcontribute to emission, reducing the emission efficiency. In addition,significant differences in the carrier concentration between well layerslead to variations in the emission wavelength and variations in the gainpeak (wavelength), which also causes a reduction in the emissionefficiency and an increase in the threshold current.

CITATION LIST Patent Literature

-   Patent Literature 1: JP 2000-174328A

Non-Patent Literature

-   Non-Patent Literature 1: IEEE, Journal of Selected Topics in Quantum    Electronics Vol. 15 No. 5 (2011) p. 1390

DISCLOSURE OF INVENTION Technical Problem

A technology in which a tunnel barrier layer is formed to reduce thedifference between the electron and hole concentrations of well layersis disclosed, for example, in JP 2000-174328A. Specifically, in thetechnology disclosed in this patent application publication, thethickness of the tunnel barrier layer is controlled to change thetunneling probability in the tunnel barrier layer. However, when theeffective mass difference between electrons and holes is great,non-uniformity of carriers is not fully avoided even though such atunnel barrier layer is provided. Reducing the thickness of only thebarrier layer without forming a tunnel barrier layer can also beconsidered. However, reducing the thickness of the barrier layer causesthe problem of a reduction in the emission efficiency of an adjacentwell layer. For example, in a light emitting device (opticalsemiconductor device) having an emission wavelength of 520 nm, it isknown that the emission efficiency when the thickness of the barrierlayer is 2.5 nm is about ¼ of the emission efficiency when the thicknessof the barrier layer is 10 nm.

Therefore, an object of the present disclosure is to provide an opticalsemiconductor device having a configuration and a structure which makeit possible to suppress a reduction in the emission efficiency and anincrease in the threshold current.

Solution to Problem

According to a first aspect, a second aspect, or a third aspect of thepresent disclosure to achieve the above object, there is provided anoptical semiconductor device including a laminate structural body inwhich an n-type compound semiconductor layer, an active layer, and ap-type compound semiconductor layer are laminated in this order. Theactive layer includes a multiquantum well structure including a tunnelbarrier layer.

In the optical semiconductor device according to the first aspect of thepresent disclosure, a compositional variation of a well layer adjacentto the p-type compound semiconductor layer is greater than acompositional variation of another well layer.

Further, in the optical semiconductor device according to the secondaspect of the present disclosure, band gap energy of the well layeradjacent to the p-type compound semiconductor layer may be smaller thanband gap energy of the other well layer.

In addition, in the optical semiconductor device according to the thirdaspect of the present disclosure, a thickness of the well layer adjacentto the p-type compound semiconductor layer may be greater than athickness of the other well layer.

Advantageous Effects of Invention

In the optical semiconductor device according to the first to thirdaspects of the present disclosure, as tunnel barrier layers areprovided, electrons are distributed unevenly such that many electronsare present at the side of the p-type compound semiconductor layer. As aresult, the emission peak wavelength or the optical gain peak wavelengthof a well layer adjacent to the p-type compound semiconductor layerdiffers from that of another well layer. Specifically, these wavelengthsof the well layer adjacent to the p-type compound semiconductor layerare shortened. The compositional variation of a well layer adjacent tothe p-type compound semiconductor layer is greater than thecompositional variation of another well layer in the opticalsemiconductor device according to the first aspect of the presentdisclosure, the band gap energy of a well layer adjacent to the p-typecompound semiconductor layer is smaller than the band gap energy ofanother well layer in the optical semiconductor device according to thesecond aspect of the present disclosure, and the thickness of a welllayer adjacent to the p-type compound semiconductor layer is greaterthan the thickness of another well layer in the optical semiconductordevice according to the third aspect of the present disclosure.Therefore, it is possible to uniformize the emission peak wavelengths orthe optical gain peak wavelengths of well layers or to reduce thedifference therebetween. As a result, it is possible to increase theemission efficiency and to reduce the threshold current. Theadvantageous effects described in this specification are only exemplaryand are not intended to be limitative and additional advantageouseffects may also be achieved.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are a schematic partial sectional view of an opticalsemiconductor device of embodiment 1 and a schematic structural view ofa multiquantum well structure in an active layer.

FIGS. 2A and 2B are schematic partial sectional views of an opticalsemiconductor device of embodiment 5 and a modified example thereof.

FIGS. 3A, 3B, and 3C are schematic partial sectional views of asubstrate or the like for explaining a method of manufacturing asurface-emitting laser device of embodiment 5.

FIGS. 4A and 4B are schematic partial sectional views of asurface-emitting laser device of embodiment 6 and a modified examplethereof, respectively.

FIGS. 5A and 5B are schematic partial sectional views of a laminatestructural body or the like for explaining a method of manufacturing asurface-emitting laser device of embodiment 6.

FIG. 6 is a schematic partial sectional view of a surface-emitting laserdevice of embodiment 7.

FIGS. 7A and 7B are schematic partial sectional views of asurface-emitting laser device of embodiments 8 and 9, respectively.

FIGS. 8A, 8B, and 8C are schematic partial sectional views of a laminatestructural body or the like for explaining a method of manufacturing asurface-emitting laser device of embodiment 8.

FIGS. 9A and 9B are schematic partial sectional views of a laminatestructural body or the like for explaining a method of manufacturing asurface-emitting laser device of embodiment 7, subsequent to FIG. 8C.

FIG. 10 is a schematic partial sectional view of a laminate structuralbody or the like for explaining a method of manufacturing asurface-emitting laser device of embodiment 7, subsequent to FIG. 9B.

FIGS. 11A and 11B are schematic partial sectional views of asurface-emitting laser device of embodiment 10.

FIG. 12 is a graph illustrating the relationship between an emissionrecombination time and the time required for a carrier to escape from awell layer.

MODE(S) FOR CARRYING OUT THE INVENTION

Although the present disclosure will hereinafter be described on thebasis of embodiments with reference to the drawings, the presentdisclosure is not limited to the embodiments and the various values ormaterials in the embodiments are exemplary. The description will begiven in the following order.

1. Optical semiconductor device according to first to third aspects ofthe present disclosure, general description

2. Embodiment 1 (optical semiconductor device according to first aspectof the present disclosure, semiconductor laser device)

3. Embodiment 2 (optical semiconductor device according to second aspectof the present disclosure, semiconductor laser device)

4. Embodiment 3 (optical semiconductor device according to third aspectof the present disclosure, semiconductor laser device)

5. Embodiment 4 (modification of embodiments 1 to 3, light emittingdiode)

6. Embodiment 5 (modification of embodiments 1 to 3, surface-emittinglaser device)

7. Embodiment 6 (modification of embodiment 5)

8. Embodiment 7 (another modification of embodiments 5 and 6)

9. Embodiment 8 (modification of embodiment 6)

10. Embodiment 9 (another modification of embodiment 8)

11. Embodiment 10 (modification of embodiments 8 and 9)

12. Others

<Optical Semiconductor Device According to First to Third Aspects of thePresent Disclosure, General Description>

In an optical semiconductor device according to the first to thirdaspects of the present disclosure, a surface of an n-type compoundsemiconductor layer that is in contact with an active layer is referredto as a second surface of the n-type compound semiconductor layer and asurface thereof that is opposite to the second surface is referred to asa first surface of the n-type compound semiconductor layer. A surface ofa p-type compound semiconductor layer that is in contact with the activelayer is referred to as a first surface of the p-type compoundsemiconductor layer and a surface thereof that is opposite to the firstsurface is referred to as a second surface of the p-type compoundsemiconductor layer.

The optical semiconductor device according to the first aspect of thepresent disclosure may be provided in a form such that a band gap energyof a well layer adjacent to the p-type compound semiconductor layer issmaller than a band gap energy of another well layer. As an alternative,the optical semiconductor device may be provided in a form such that thethickness of a well layer adjacent to the p-type compound semiconductorlayer is greater than the thickness of another well layer and, in thiscase, band gap energy of the well layer adjacent to the p-type compoundsemiconductor layer may be smaller than band gap energy of the otherwell layer.

The optical semiconductor device according to the second aspect of thepresent disclosure may be provided in a form such that the thickness ofa well layer adjacent to the p-type compound semiconductor layer isgreater than the thickness of another well layer.

The optical semiconductor device according to the first to third aspectsof the present disclosure including the various preferable formsdescribed above may be provided in a form such that a tunnel barrierlayer is formed between a well layer and a barrier layer. As an example,when an active layer includes two well layers and one barrier layer, theactive layer has a structure which includes, sequentially from the sideof the n-type compound semiconductor layer, a first well layer, a firsttunnel barrier layer, the barrier layer, a second tunnel barrier layer,and a second well layer. However, the number of well layers included inthe active layer is not limited to 2 and may be equal to or greater than3.

In the optical semiconductor device according to the first to thirdaspects of the present disclosure including the various preferable formsdescribed above, it is preferable that the thickness of each of thetunnel barrier layers is equal to or less than 4 nm. The lower limit ofthe thickness of each tunnel barrier layer is not especially limited aslong as the tunnel barrier layer is formed. The thickness of each tunnelbarrier layer may be constant and may also vary.

In addition, the optical semiconductor device according to the first tothird aspects of the present disclosure including the various preferableforms described above may be configured such that the active layer ismade of an AlInGaN-based compound semiconductor. In this case, thetunnel barrier layers may be made of GaN. In these cases, the n-typecompound semiconductor layer may be formed on a c-surface of a GaNsubstrate. In addition, in these cases, the emission wavelength may beequal to or greater than 440 nm.

The compositional variation or the composition of the well layer in theoptical semiconductor device according to the first aspect of thepresent disclosure may be measured, for example, on the basis of a 3dimensional atom probe (3DAP). When the active layer is made of anAlInGaN-based compound semiconductor, the compositional variation or thecomposition of indium the may be measured on the basis of a 3dimensional atom probe. With respect to the 3 dimensional atom probe,for example, seehttp://www.nanoanalysis.co.jp/business/case_example_49.html. The indiumcomposition can be measured and indium atoms in the composition can becounted using the 3 dimensional atom probe. In the case where the indiumcomposition and the number of indium atoms in the composition arerepresented by a histogram or the like with the horizontal axisindicating the composition of indium and the vertical axis indicatingthe number of indium atoms in the composition, it is known that thecompositional variation of a well layer adjacent to the p-type compoundsemiconductor layer is greater than the compositional variation ofanother well layer when values such as a half-value width, a variance,or a standard deviation of the histogram of the well layer adjacent tothe p-type compound semiconductor layer are greater than those valuesfor the other well layer.

The band gap energy of the optical semiconductor device according to thesecond aspect of the present disclosure can be determined, for example,on the basis of an average value of the composition of indium measuredby the 3 dimensional atom probe and the thickness of a well layer of theoptical semiconductor device according to the third aspect of thepresent disclosure can be obtained, for example, using a high-resolutionelectron microscope or the like.

In the optical semiconductor device according to the second aspect ofthe present disclosure or in the preferable forms of the opticalsemiconductor device according to the first and third aspects of thepresent disclosure, the band gap energy of the well layer adjacent tothe p-type compound semiconductor layer may be smaller than the band gapenergy of the other well layer and a value obtained by subtracting themaximum value of the band gap energy of the other well layer from theband gap energy of the well layer adjacent to the p-type compoundsemiconductor layer may be exemplified as, but is not limited to, 1×10⁻⁴eV to 2×10⁻¹ eV.

In the optical semiconductor device according to the third aspect of thepresent disclosure or in the preferable forms of the opticalsemiconductor device according to the first and second aspects of thepresent disclosure, the thickness of the well layer adjacent to thep-type compound semiconductor layer may be greater than the thickness ofthe other well layer and a value obtained by subtracting the maximumvalue of the thickness of the other well layer from the thickness of thewell layer adjacent to the p-type compound semiconductor layer may beexemplified as, but is not limited to, 0.05 nm to 2 nm.

As a material which constitutes the active layer and as a material whichconstitutes a laminate structural body in the optical semiconductordevice according to the first to third aspects of the presentdisclosure, it is possible to employ a GaN-based compound semiconductor,specifically an AlInGaN-based compound semiconductor as described above,and more specifically, GaN, AlGaN, InGaN, and AlInGaN. In addition, aboron (B) atom(s), a thallium (TI) atom(s), an arsenic (As) atom(s), aphosphorus (P) atom(s), or an antimony (Sb) atom(s) may be included asdesired in such compound semiconductors. A combination (a compoundsemiconductor that constitutes a well layer, a compound semiconductorthat constitutes a barrier layer) may be exemplified by(In_(y)Ga_((1-y))N, In_(z)Ga_((1-z))N) (where y>z) and(In_(y)Ga_((1-y))N, AlGaN). The thickness of a well layer may be, but isnot limited to, equal to or greater than 1 nm and equal to or less than10 nm, preferably equal to or greater than 1 nm and equal to or lessthan 8 nm, and the impurity doping concentration of a barrier layer maybe, but is not limited to, equal to or greater than 1×10¹⁸ cm⁻³ andequal to or less than 1×10²⁰ cm⁻³, preferably equal to or greater than1×10¹⁸ cm⁻³ and equal to or less than 1×10¹⁹ cm⁻³.

In the p-type compound semiconductor layer, an electron barrier layermay be formed near or adjacent to the active layer and a non-dopedcompound semiconductor layer (for example, a non-doped InGaN layer or anon-doped AlGaN layer) may be formed between the active layer and theelectron barrier layer. In addition, a non-doped InGaN layer may beformed as a light guide layer between the active layer and the non-dopedcompound semiconductor layer. The p-type compound semiconductor layermay also has a structure such that the top layer of the p-type compoundsemiconductor layer is occupied by a Mg doped GaN layer (a p-sidecontact layer or a p-contact layer). The electron barrier layer, thenon-doped compound semiconductor layer, the light guide layer, and thep-side contact layer (p-contact layer) constitute the p-type compoundsemiconductor layer.

The n-type compound semiconductor layer is formed preferably, but notlimited to being formed, on a c-surface of a GaN substrate, i.e., on a{0001} surface thereof and may also be formed on a non-polar surfacesuch as an a-surface which is an {11-20} surface, an m-surface which isa {1-100} surface, or a {1-102} surface or alternatively on a half-polarsurface such as {11-2n} surfaces including an {11-24} surface and an{11-22} surface, a {10-11} surface, a {10-12} surface, or a {20-21}surface. A foundation layer or a buffer layer may also be formed on asurface (primary surface) of a substrate other than the GaN substrate,such as a sapphire substrate, a GaAs substrate, a GaN substrate, a SiCsubstrate, an alumina substrate, a ZnS substrate, a ZnO substrate, anAlN substrate, a LiMgO substrate, a LiGaO₂ substrate, a MgAl₂O₄substrate, an InP substrate, or a Si substrate.

As a method of forming various compound semiconductor layers (forexample, GaN-based compound semiconductor layers) that constitute theoptical semiconductor device of the present disclosure, it is possibleto employ a metalorganic vapor phase growth method (such as an MOCVDmethod or an MOVPE method), a molecular beam epitaxy method (i.e., anMBE method), a hydride vapor phase growth method in which halogencontributes to transportation or reaction, or the like.

Here, in the MOCVD method, it is possible to employ trimethylgallium(TMG) gas or triethylgallium (TEG) gas as an organic gallium source gasand it is possible to employ ammonia gas or hydrazine gas as a nitrogensource gas. For example, silicon (Si) may be added as an n-type impurity(n-type dopant) to form a GaN-based compound semiconductor layer havingn-type conductivity and, for example, magnesium (Mg) may be added as ap-type impurity (p-type dopant) to form a GaN-based compoundsemiconductor layer having p-type conductivity. In the case wherealuminum (Al) or indium (In) is included as constituent atoms in theGaN-based compound semiconductor layer, trimethylaluminum (TMA) gas maybe used as an Al source and trimethylindium (TMI) gas may be used as anIn source. In addition, monosilane gas (SiH₄ gas) may be used as a Sisource, and cyclopentadienyl magnesium gas, methyl cyclopentadienylmagnesium gas, or bis(cyclopentadienyl)magnesium (Cp₂Mg) gas may be usedas a Mg source. As an n-type impurity (n-type dopant) other than Si, itis possible to employ Ge, Se, Sn, C, Te, S, O, Pd, or Po and, as ap-type impurity (p-type dopant) other than Mg, it is possible to employZn, Cd, Be, Ca, Ba, C, Hg, or Sr.

In the optical semiconductor device, a p-side electrode is formed on thep-type compound semiconductor layer (which is a compound semiconductorlayer having p-type conductivity). Here, the p-side electrode may bemade, for example, of a single palladium (Pd) layer, a single nickel(Ni) layer, a single platinum (Pt) layer, a transparent conductivematerial layer such as an ITO layer, a laminate structure of palladiumlayer/platinum layer where the palladium layer is in contact with thep-type compound semiconductor layer, or a laminate structure ofpalladium layer/nickel layer where the palladium layer is in contactwith the p-type compound semiconductor layer. In the case where a lowermetal layer is made of palladium and an upper metal layer is made ofnickel, it is desirable that the thickness of the upper metal layer beequal to or greater than 0.1 μm, preferably equal to or greater than 0.2μm. As an alternative, the p-side electrode is preferably made of asingle palladium (Pd) layer. In this case, it is desirable that thethickness be equal to or greater than 20 nm, preferably equal to orgreater than 50 nm. As another alternative, the p-side electrode ispreferably made of a single palladium (Pd) layer, a single nickel (Ni)layer, a single platinum (Pt) layer, or a laminate structure including alower metal layer and an upper metal layer where the lower metal layeris in contact with the p-type compound semiconductor layer.

It is desirable that the n-side electrode that is electrically connectedto the n-type compound semiconductor layer (which is a compoundsemiconductor layer having n-type conductivity) have a single layerconfiguration or a multilayer configuration including at least one typeof metal selected from the group consisting of gold (Au), silver (Ag),palladium (Pd), aluminum (Al), titanium (Ti), tungsten (W), copper (Cu),zinc (Zn), tin (Sn), and indium (In), which may be exemplified, forexample, by Ti/Au, Ti/Al, or Ti/Pt/Au. The form in which the n-sideelectrode is electrically connected to the n-type compound semiconductorlayer includes a form in which the n-side electrode is formed on then-type compound semiconductor layer and a form in which the n-sideelectrode is connected to the n-type compound semiconductor layer via aconductive substrate or base or a conductive material layer. The n-sideelectrode or the p-side electrode may be deposited, for example, usingvarious PVD methods such as a vacuum evaporation method or a sputteringmethod.

A pad electrode may be provided on the n-side electrode or the p-sideelectrode to electrically connect the n-side electrode or the p-sideelectrode to an external electrode or circuit. It is desirable that thepad electrode have a single layer configuration or a multilayerconfiguration including at least one type of metal selected from thegroup consisting of titanium (Ti), aluminum (Al), platinum (Pt), gold(Au), and nickel (Ni). As an alternative, the pad electrode may have amultilayer configuration which is exemplified by a Ti/Pt/Au multilayerconfiguration and a Ti/Au multilayer configuration.

As the optical semiconductor device, it is possible to employ a lightemitting diode (LED), a semiconductor laser device (LD), a superluminescent diode (SLD), a surface-emitting laser device which is calleda vertical resonator laser or a VCSEL, or a semiconductor opticalamplifier (SOA). These optical semiconductor devices or the basicconfigurations and structures thereof may have well-known configurationsand structures, except for the configuration and structure of the activelayer. When a light emitting diode (LED) is employed, light generatedfrom the active layer may be emitted outside via the n-type compoundsemiconductor layer and may be emitted outside via the p-type compoundsemiconductor layer. When a semiconductor laser device (LD) is employed,light generated from the active layer is emitted outside from an endsurface of the laminate structural body. That is, a resonator isconfigured by optimizing the optical reflectance of a first end surfaceof the laminate structural body and the optical reflectance of a secondend surface that is opposite to the first end surface, and light isemitted from the first end surface. As an alternative, an externalresonator may be provided and a monolithic type semiconductor laserdevice may be employed. The external resonator type semiconductor laserdevice may be a light collecting type and may be a collimation type.When a super luminescent diode (SLD) is employed, the opticalreflectance of the first end surface of the laminate structural body ismade very low and the optical reflectance of the second end surface ismade very high and no resonator is constructed, and light generated fromthe active layer is emitted from the first end surface. Ananti-reflective coating layer (AR) or a low-reflective coating layer isformed on the first end surface and a high-reflective coating layer (HR)is formed on the second end surface. A semiconductor optical amplifier(SOA) is configured to amplify an optical signal in a state of directlight without conversion into an electrical signal and has a laserstructure that fully excludes resonator effects and amplifies incidentlight with an optical gain of the semiconductor optical amplifier. Whena semiconductor optical amplifier is employed, the optical reflectanceof the first end surface of the laminate structural body and the opticalreflectance of the second end surface thereof are made very low and noresonator is constructed, and light incident on the second end surfaceis amplified and emitted from the first end surface. When asurface-emitting laser device (VCSEL) is employed, a first opticalreflective layer is formed on a first surface of the n-type compoundsemiconductor layer and a second optical reflective layer is formed onor over a second surface of the p-type compound semiconductor layer. Ananti-reflective coating layer (AR) or a low-reflective coating layer isformed on the first end surface and the second end surface. As theanti-reflective coating layer (low-reflective coating layer) or thehigh-reflective coating layer, it is possible to employ a laminatestructure of at least two layers selected from the group consisting of atitanium oxide layer, a tantalum oxide layer, a zirconium oxide layer, asilicon oxide layer, an aluminum oxide layer, an aluminum nitride layer,and a silicon nitride layer and they may be formed on the basis of a PVDmethod such as a sputtering method or a vacuum evaporation method.

In the case where the laminate structural body of the opticalsemiconductor device of the present disclosure has a ridge stripestructure, the ridge stripe structure may be constructed of a part ofthe p-type compound semiconductor layer in the thickness directionthereof and may be constructed of the p-type compound semiconductorlayer alone, may be constructed of both the p-type compoundsemiconductor layer and the active layer and may also be constructed ofa part of the p-type compound semiconductor layer, the active layer, andthe n-type compound semiconductor layer in the thickness directionthereof. To form the ridge stripe structure, a compound semiconductorlayer may be patterned, for example, using a dry etching method.

Embodiment 1

Embodiment 1 relates to an optical semiconductor device, specifically toa semiconductor laser device (LD), according to the first aspect of thepresent disclosure. FIG. 1A is a schematic partial sectional view of anoptical semiconductor device of embodiment 1, specifically a schematicpartial sectional view of an optical semiconductor device taken along avirtual plane (YZ plane) perpendicular to the extending direction of aresonator and FIG. 1B is a schematic structural view of a multiquantumwell structure in an active layer. FIG. 1B shows a band structurewithout considering the influence of piezoelectric fields for the sakeof convenience.

The semiconductor optical device of embodiment 1 or embodiments 1 to 10described later has a laminate structural body 20 in which an n-typecompound semiconductor layer 21, an active layer 23, and a p-typecompound semiconductor layer 22 are laminated in this order. The activelayer 23 includes a multiquantum well structure having a tunnel barrierlayer 33. Specifically, the tunnel barrier layer 33 is formed between awell layer 31 and a barrier layer 32. In embodiment 1 or embodiments 2to 10 described later, the active layer 23 includes two well layers 31 ₁and 31 ₂ and one barrier layer 32. More specifically, the active layer23 has a multiquantum well structure which includes, sequentially fromthe side of the n-type compound semiconductor layer 21, a first welllayer 31 ₁, a first tunnel barrier layer 331, a barrier layer 32, asecond tunnel barrier layer 332, and a second well layer 31 ₂. Thethickness of each of the tunnel barrier layers 331 and 332 is equal toor less than 4 nm.

The laminate structural body 20 is formed on a substrate 11. Thesubstrate 11 is specifically constructed of a GaN substrate and thelaminate structural body 20, specifically the n-type compoundsemiconductor layer 21, is formed on a c-surface of the GaN substrate,i.e., on a (0001) surface thereof. An n-side electrode 25 is formed on arear surface of the substrate 11 and a p-side electrode 26 is formed onthe p-type compound semiconductor layer 22. The laminate structural body20 is covered with an insulating layer 24. The refractive index of amaterial that constitutes the insulating layer 24 is preferably lessthan the refractive index of a material that constitutes the laminatestructural body 20. The material that constitutes the insulating layer24 may be exemplified by SiOx-based material including SiO₂, aSiN_(X)-based material, a SiO_(X)N_(Z)-based material, TaO_(X), ZrO_(X),AlN_(X), AlO_(X), and GaO_(X) or may include an organic material such asa polyimide resin. Examples of the method of forming the insulatinglayer 24 include a PVD method such as a vacuum evaporation method or asputtering method or a CVD method, and the insulating layer 24 may alsobe formed on the basis of a coating method.

Here, in the optical semiconductor device of embodiment 1 or embodiments2 to 4 described later which is made of a semiconductor laser device, itis assumed that the configuration of the laminate structural body 20 orthe like is as in Table 1 below. It is also assumed that theconfiguration of the active layer 23 of the optical semiconductor deviceof embodiment 1 is as in Table 2. In addition, the value of thecomposition of indium in the two tunnel barrier layers 331 and 332 maybe made less than the value of the composition of indium in the barrierlayer 32. The active layer 23 is made of an AlInGaN-based compoundsemiconductor and the tunnel barrier layers 331 and 332 are made of GaN.The emission wavelength of the semiconductor laser device of embodiment1 is equal to or greater than 440 nm and is specifically 460 nm.

TABLE 1 p-side electrode 26 Pd/Au p-type compound semiconductor layer 22p-contact layer 22C GaN (thickness: 0.1 μm) p-clad layer 22B AlGaN(thickness: 0.3 μm) Electron barrier layer 22A AlGaN Active layer 23 SeeTable 2 n-type compound semiconductor layer 21 n-guide layer 21B InGaN(thickness: 0.1 μm) n-clad layer 21A AlGaN (thickness: 0.4 μm) Substrate11 GaN substrate, c-surface n-side electrode 25 Ti/Pt/Au

TABLE 2 Active layer Second well layer In_(0.30)Ga_(0.70)N (thickness:2.5 nm) Second tunnel barrier layer GaN (thickness: 2.0 nm) Barrierlayer In_(0.05)Ga_(0.95)N (thickness: 4.0 nm) First tunnel barrier layerGaN (thickness: 2.0 nm) First well layer In_(0.30)Ga_(0.70)N (thickness:2.5 nm)

Here, in the optical semiconductor device of embodiment 1, thecompositional variation of a well layer adjacent to the p-type compoundsemiconductor layer is greater than the compositional variation ofanother well layer. Specifically, the variation of the composition ofindium in the well layers 31 ₁ and 31 ₂ is increased by allowing thedeposition speed or temperature and/or the deposition pressure of thefirst well layer 31 ₁ to be different from the deposition speed ortemperature and/or the deposition pressure of the second well layer 31 ₂when the laminate structural body 20 is deposited on the basis of anMOCVD method. The compositional variation or the composition of indiumcan be measured on the basis of a 3 dimensional atom probe (3DAP) asdescribed above. Specifically, in the case where the indium compositionand the number of indium atoms in the composition measured by the 3dimensional atom probe are represented by a histogram or the like withthe horizontal axis indicating the indium composition and the verticalaxis indicating the number of indium atoms in the composition, it isfound that the compositional variation of a well layer adjacent to thep-type compound semiconductor layer is greater than the compositionalvariation of another well layer when a half-value width of the histogramof the well layer adjacent to the p-type compound semiconductor layer isgreater than a half-value width of the other well layer.

The optical semiconductor device of embodiment 1 or embodiments 2 and 3described later can be manufactured using the following method.

[Process 100]

First, a laminate structural body 20 including an n-type compoundsemiconductor layer 21, an active layer 23, and a p-type compoundsemiconductor layer 22 which are sequentially laminated is formed on asubstrate 11, specifically on a (0001) surface of an n-type GaNsubstrate, on the basis of a well-known MOCVD method. Then, the p-typecompound semiconductor layer 22 is partially etched in the thicknessdirection to form a ridge stripe structure 27. The thickness of a partof the p-type compound semiconductor layer 22 which constitutes theridge stripe structure 27 is 0.12 μm.

[Process 110]

Then, an insulating layer 24 including SiO₂ is formed to cover thep-type compound semiconductor layer 22 and thereafter a Si layer (notshown) is formed on the insulating layer 24. Then, a p-side electrode 26is formed on the p-type compound semiconductor layer 22 after portionsof the insulating layer 24 and the Si layer where the p-side electrode26 is to be formed are removed. Specifically, after a p-side electrodelayer is deposited over the entire surface on the basis of a vacuumevaporation method, an etching resist layer is formed on the p-sideelectrode layer on the basis of a photolithography technology. Then, theetching resist layer is removed after a portion of the p-side electrodelayer which is not covered with the etching resist layer is removed onthe basis of an etching method. The p-side electrode 26 may also beformed on the p-type compound semiconductor layer 22 on the basis of aliftoff method.

[Process 120]

Then, the substrate 11 is polished from the rear surface to decrease thethickness of the substrate 11. Thereafter, an n-side electrode 25 isformed on the rear surface of the substrate 11 and a pad electrode isformed on the p-side electrode 26. Then, cleavage or the like of thesubstrate 11 is performed, and an anti-reflective coating layer (AR) ora low-reflective coating layer is formed on a first end surface of thelaminate structural body 20 and a high-reflective coating layer (HR) isformed on a second end surface thereof to perform optical reflectancecontrol on the first and second end surfaces of the laminate structuralbody 20. Packaging may then be performed to manufacture an opticalsemiconductor device.

In the optical semiconductor device of embodiment 1 or embodiments 2 to10 described later, as tunnel barrier layers are provided, electrons aredistributed unevenly such that many electrons are present at the side ofthe p-type compound semiconductor layer. As a result, the emission peakwavelength or the optical gain peak wavelength of a well layer adjacentto the p-type compound semiconductor layer differs from that of anotherwell layer. Specifically, these wavelengths of the well layer adjacentto the p-type compound semiconductor layer are shortened. However, inthe optical semiconductor device of embodiment 1, the compositionalvariation of a well layer adjacent to the p-type compound semiconductorlayer is greater than the compositional variation of another well layerand therefore the emission peak wavelength or the optical gain peakwavelength of the well layer adjacent to the p-type compoundsemiconductor layer is lengthened such that it is possible to uniformizethe emission peak wavelengths or the optical gain peak wavelengths ofwell layers or to reduce the difference therebetween. As a result, it ispossible to increase the emission efficiency and to reduce the thresholdcurrent. It is also possible to suppress non-uniformity of carriers inthe active layer since it is possible to exclude the influence ofpiezoelectric fields on the well layer or barrier layer even though theactive layer is formed on the c-surface of the GaN substrate.

Note that, as described above, the configuration and structure of asuper luminescent diode (SLD) or a semiconductor optical amplifier (SOA)are substantially the same as those of the semiconductor laser devicedescribed above in embodiment 1 or a semiconductor laser devicedescribed later in embodiments 2 and 3, except that the optimization ofoptical reflectance of the first and second end surfaces, the formationof a resonator, or the like thereof differ from that of thesemiconductor laser device.

Embodiment 2

Embodiment 2 relates to an optical semiconductor device, specifically toa semiconductor laser device (LD), according to the second aspect of thepresent disclosure. In the optical semiconductor device of embodiment 2,the band gap energy of a well layer (the second well layer 31 ₂)adjacent to the p-type compound semiconductor layer is smaller than theband gap energy of another well layer (specifically, the first welllayer 31 ₁) (see Table 4). Here, it is assumed that the configuration ofan active layer 23 in the optical semiconductor device of embodiment 2is as in Table 3. The emission wavelength of the semiconductor laserdevice of embodiment 2 is equal to or greater than 440 nm and isspecifically 460 nm. Specifically, the band gap energy of the well layer(the second well layer 31 ₂) adjacent to the p-type compoundsemiconductor layer can be made smaller than the band gap energy ofanother well layer (specifically, the first well layer 31 ₁) by allowingthe amount of supply of trimethylindium (TMI) gas as an indium sourcefor depositing the second well layer 31 ₂ to be greater than the amountof supply of trimethylindium gas as an indium source for depositing thefirst well layer 31 ₁ or by increasing the deposition speed of thesecond well layer 31 ₂ when the laminate structural body 20 is depositedon the basis of an MOCVD method.

TABLE 3 Active layer Second well layer In_(0.19)Ga_(0.81)N (thickness:2.5 nm) Second tunnel barrier layer GaN (thickness: 2.0 nm) Barrierlayer In_(0.04)Ga_(0.96)N (thickness: 4.0 nm) First tunnel barrier layerGaN (thickness: 2.0 nm) First well layer In_(0.18)Ga_(0.82)N (thickness:2.5 nm)

TABLE 4 Band gap energy of second well layer 31₂ 2.695 eV Band gapenergy of first well layer 31₁ 2.654 eV

In the optical semiconductor device of embodiment 2, the band gap energyof a well layer adjacent to the p-type compound semiconductor layer ismade smaller than the band gap energy of another well layer andtherefore it is possible to uniformize the emission peak wavelengths orthe optical gain peak wavelengths of well layers or to reduce thedifference therebetween. As a result, it is possible to increase theemission efficiency and to reduce the threshold current.

Embodiment 3

Embodiment 3 relates to an optical semiconductor device, specifically toa semiconductor laser device (LD), according to the third aspect of thepresent disclosure. In the optical semiconductor device of embodiment 3,the thickness of a well layer (the second well layer 31 ₂) adjacent tothe p-type compound semiconductor layer is greater than the thickness ofanother well layer (specifically, the first well layer 31 ₁). Here, itis assumed that the configuration of an active layer 23 in the opticalsemiconductor device of embodiment 3 is as in Table 5. The emissionwavelength of the semiconductor laser device of embodiment 3 is equal toor greater than 440 nm and is specifically 460 nm. Specifically, thethickness of the well layer (the second well layer 31 ₂) adjacent to thep-type compound semiconductor layer can be made greater than thethickness of another well layer (specifically, the first well layer 31₁) by allowing the deposition time of the second well layer 31 ₂ to belonger than the deposition time of the first well layer 31 ₁ or byincreasing the deposition speed of the second well layer 31 ₂ when thelaminate structural body 20 is deposited on the basis of an MOCVDmethod.

TABLE 5 Active layer Second well layer In_(0.18)Ga_(0.82)N (thickness:2.8 nm) Second tunnel barrier layer GaN (thickness: 2.0 nm) Barrierlayer In_(0.05)Ga_(0.95)N (thickness: 4.0 nm) First tunnel barrier layerGaN (thickness: 2.0 nm) First well layer In_(0.18)Ga_(0.82)N (thickness:2.5 nm)

In the optical semiconductor device of embodiment 3, the thickness of awell layer adjacent to the p-type compound semiconductor layer is madegreater than the thickness of another well layer and therefore it ispossible to uniformize the emission peak wavelengths or the optical gainpeak wavelengths of well layers or to reduce the differencetherebetween. As a result, it is possible to increase the emissionefficiency and to reduce the threshold current.

Note that it is also possible to combine embodiments 1 and 2, to combineembodiments 1 and 3, to combine embodiments 2 and 3, and to combineembodiments 1, 2, and 3.

Embodiment 4

Embodiment 4 is a modification of embodiments 1 to 3 and specificallyrelates to a light emitting diode (LED). The configuration (composition)of a laminate structural body in an optical semiconductor device ofembodiment 4 may be the same as the configuration (composition) of thelaminate structural body in the optical semiconductor device ofembodiments 1 to 3 shown in Tables 1 to 5. The structure of the opticalsemiconductor device of embodiment 4 may be substantially the same asthe structure of the optical semiconductor device described inembodiments 1 to 3, except that light generated from an active layer isemitted outside via an n-type compound semiconductor layer 21 or via ap-type compound semiconductor layer 22 and there is no need to form theridge stripe structures and therefore a detailed description thereof isomitted.

Embodiment 5

Embodiment 5 is also a modification of embodiments 1 to 3 andspecifically relates to a surface-emitting laser device (verticalresonator laser or VCSEL). The surface-emitting laser device ofembodiment 5 or embodiments 6 and 7 described later is described as asurface-emitting laser device in which an n-type compound semiconductorlayer is formed on a substrate including a first optical reflectivelayer formed thereon on the basis of lateral growth using a method ofepitaxial growth in the lateral direction such as an epitaxial lateralovergrowth (ELO) method, but is not limited to this form ofsurface-emitting laser device. In the following description, an “opticalsemiconductor device” may sometimes be referred to as a“surface-emitting laser device”.

As shown in the schematic partial sectional view of FIG. 2A, thesurface-emitting laser device of embodiment 5 or embodiments 6 to 10described later includes: a first optical reflective layer 51;

-   -   a laminate structural body 20 including an n-type compound        semiconductor layer 21, an active layer 23, and a p-type        compound semiconductor layer 22 formed on the first optical        reflective layer 51; and a p-side electrode 42 and a second        optical reflective layer 52 formed on the p-type compound        semiconductor layer 22.

The first optical reflective layer 51 is formed on a first surface 21 aof the n-type compound semiconductor layer 21 and the second opticalreflective layer 52 is formed on a second surface 22 b of the p-typecompound semiconductor layer 22. The second optical reflective layer 52is opposite to the first optical reflective layer 51.

Various forms of the surface-emitting laser device of embodiment 5 orembodiments 6 to 10 described later are described below.

The plan shape of the first optical reflective layer may be variouspolygonal shapes including a regular hexagon, a circle, an ellipse, agrid (rectangular shape), an island, or a stripe. The cross-sectionalshape of the first optical reflective layer may be rectangular but ismore preferably trapezoidal. That is, it is more preferable that theside surface of the first optical reflective layer have a forwardtapered shape.

In the surface-emitting laser device of embodiment 5 or embodiments 6and 7 described later, the substrate may remain. As an alternative, inthe surface-emitting laser device of embodiment 5 or embodiments 6 and 7described later, the substrate may be removed after an active layer, ap-type compound semiconductor layer, a p-side electrode, and a secondoptical reflective layer are sequentially formed on an n-type compoundsemiconductor layer. Specifically, after the active layer, the p-typecompound semiconductor layer, the p-side electrode, and the secondoptical reflective layer are sequentially formed on the n-type compoundsemiconductor layer and the second optical reflective layer is thenfixed to a support substrate, the substrate may be removed (for example,using the first optical reflective layer as a polishing stopper layer insome cases) to expose the n-type compound semiconductor layer (the firstsurface of the n-type compound semiconductor layer) and also to exposethe first optical reflective layer in the case where the first opticalreflective layer has been previously formed. The n-side electrode maythen be formed on the n-type compound semiconductor layer (on the firstsurface of the n-type compound semiconductor layer). In the case wherethe substrate remains, the n-side electrode may be formed on a rearsurface of the substrate.

In the case where the substrate is constructed of a GaN substrate,removal of the GaN substrate may be performed in a manner based on achemical/mechanical polishing method (CMP method). First, a part of theGaN substrate may be removed or the thickness of the GaN substrate maybe reduced using a wet etching method, which uses an alkaline aqueoussolution such as an aqueous solution of sodium hydroxide or an aqueoussolution of potassium hydroxide, an ammonia solution+hydrogen peroxidewater, a sulfuric acid solution+hydrogen peroxide water, a hydrochloricacid solution+hydrogen peroxide water, or a phosphoric acidsolution+hydrogen peroxide water, a dry etching method, a liftoff methodusing a laser, a mechanical polishing method, or the like, or acombination thereof and a chemical/mechanical polishing method may thenbe performed to expose the n-type compound semiconductor layer (thefirst surface of the n-type compound semiconductor layer) and also toexpose the first optical reflective layer in the case where the firstoptical reflective layer has been previously formed.

In the surface-emitting laser device of embodiment 5 or embodiments 6 to10 described later, the surface roughness Ra of the p-type compoundsemiconductor layer (the second surface of the p-type compoundsemiconductor layer) is preferably equal to or less than 1.0 nm. Thesurface roughness Ra is predefined in JIS B-610: 2001 and may bespecifically measured on the basis of an observation based on an AFM ora cross-sectional TEM. The distance between the first optical reflectivelayer and the second optical reflective layer is preferably equal to orgreater than 0.15 μm and equal to or less than 50 μm.

In addition, the surface-emitting laser device of embodiment 5 orembodiments 6 to 10 described later is preferably provided in a formsuch that the center of gravity of the area of the second opticalreflective layer is not on a line normal to the first optical reflectivelayer which passes through the center of gravity of the area of thefirst optical reflective layer. As an alternative, the surface-emittinglaser device is preferably provided in a form such that the center ofgravity of the area of the active layer (specifically, the center ofgravity of an area of the active layer which constitutes a deviceregion, which refers to the same in the following description) is not onthe line normal to the first optical reflective layer which passesthrough the center of gravity of the area of the first opticalreflective layer.

When the n-type compound semiconductor layer is formed on the substrateincluding the first optical reflective layer formed thereon on the basisof lateral growth using a method of epitaxial growth in the lateraldirection such as an epitaxial lateral overgrowth (ELO) method, a lot ofcrystalline defects may occur at a portion of the first opticalreflective layer where regions of the n-type compound semiconductorlayer meet as the n-type compound semiconductor layer is epitaxiallygrown from edge portions of the first optical reflective layer toward acentral portion of the first optical reflective layer. When the meetingportion having a lot of crystalline defects is positioned at a centralportion of the device region (which is described later), this may causeadverse effects on the characteristics of the surface-emitting laserdevice. It is possible to completely suppress the occurrence of adverseeffects on the characteristics of the surface-emitting laser device byemploying the form in which the center of gravity of the area of asecond optical reflective layer is not on a line normal to a firstoptical reflective layer which passes through the center of gravity ofthe area of the first optical reflective layer or the form in which thecenter of gravity of the area of the active layer is not on the linenormal to the first optical reflective layer which passes through thecenter of gravity of the area of the first optical reflective layer asdescribed above.

The surface-emitting laser device of embodiment 5 or embodiments 6 to 10described later may be provided in a form such that light generated fromthe active layer is emitted outside via the second optical reflectivelayer (which is hereinafter referred to as a “second optical reflectivelayer emitting type of surface-emitting laser device” for the sake ofconvenience) and may be provided in a form such that light generatedfrom the active layer is emitted outside via the first opticalreflective layer (which is hereinafter referred to as a “first opticalreflective layer emitting type of surface-emitting laser device” for thesake of convenience). In the first optical reflective layer emittingtype of surface-emitting laser device, the substrate may be removed insome cases as described above.

When Si represents the area of a portion of the first optical reflectivelayer which is in contact with the first surface of the n-type compoundsemiconductor layer (i.e., a portion thereof which is opposite to thesecond optical reflective layer) and S₂ represents the area of a portionof the second optical reflective layer which is opposite to the secondsurface of the p-type compound semiconductor layer (i.e., a portionthereof which is opposite to the first optical reflective layer), it isdesirable, but not necessary, that S₁>S₂ be satisfied in the case of thefirst optical reflective layer emitting type of surface-emitting laserdevice and it is desirable, but not necessary, that S₁<S₂ be satisfiedin the case of the second optical reflective layer emitting type ofsurface-emitting laser device.

Further, when S₃ represents the area of a portion of the first opticalreflective layer which is in contact with the first surface of then-type compound semiconductor layer (i.e., a portion thereof which isopposite to the second optical reflective layer) and which constitutesthe device region (described later) and S₄ represents the area of aportion of the second optical reflective layer which is opposite to thesecond surface of the p-type compound semiconductor layer (i.e., aportion thereof which is opposite to the first optical reflective layer)and which constitutes the device region in the form in which the centerof gravity of the area of the second optical reflective layer is not onthe line normal to the first optical reflective layer which passesthrough the center of gravity of the area of the first opticalreflective layer or in the form in which the center of gravity of thearea of the active layer is not on the line normal to the first opticalreflective layer which passes through the center of gravity of the areaof the first optical reflective layer, it is desirable, but notnecessary, that S₃>S₄ be satisfied in the case of the first opticalreflective layer emitting type of surface-emitting laser device and itis desirable, but not necessary, that S₃<S₄ be satisfied in the case ofthe second optical reflective layer emitting type of surface-emittinglaser device.

The surface-emitting laser device of embodiment 5 or embodiments 6 to 10described later may be provided in a form such that the second opticalreflective layer is fixed to the support substrate as described above inthe case where the substrate is removed in the first optical reflectivelayer emitting type of surface-emitting laser device. The n-sideelectrode may be formed on an exposed surface of the substrate in thecase where the substrate is not removed in the first optical reflectivelayer emitting type of surface-emitting laser device. As an arrangementof the first optical reflective layer and the n-side electrode on thefirst surface of the n-type compound semiconductor layer in the casewhere the substrate is removed, it is possible to employ an arrangementin which the first optical reflective layer and the n-side electrode arein contact with each other or alternatively an arrangement in which thefirst optical reflective layer and the n-side electrode are spaced apartfrom each other or an arrangement in which the n-side electrode is alsoformed over or under an edge portion of the first optical reflectivelayer in some cases. As an alternative, it is possible to employ aconfiguration in which the first optical reflective layer and the n-sideelectrode are spaced apart from each other, i.e., have an offsettherebetween, and the spacing distance is within 1 mm.

In addition, the surface-emitting laser device of the present disclosureincluding the various preferable forms described above andconfigurations described above may be provided in a form such that then-side electrode is made of metal, alloy, or a transparent conductivematerial and the p-side electrode is made of a transparent conductivematerial. By making the p-side electrode of a transparent conductivematerial, it is possible to spread current in the horizontal direction(i.e., in the surface direction of the p-type compound semiconductorlayer) and to efficiently supply current to the device region (which isdescribed below).

“Device region” refers to a region (current confinement region) intowhich confined current is injected, a region within which light isconfined due to a refractive index difference or the like, a region inwhich laser oscillation occurs within a region between the first opticalreflective layer and the second optical reflective layer, or a regionwhich actually contributes to laser oscillation within the regionbetween the first optical reflective layer and the second opticalreflective layer.

The surface-emitting laser device may be configured such that it is madeof a surface-emitting laser device which emits light from a top surfaceof an n-type compound semiconductor layer via a first optical reflectivelayer or may be configured such that it is made of a surface-emittinglaser device which emits light from a top surface of a p-type compoundsemiconductor layer via a second optical reflective layer.

It is preferable that a current confinement structure be formed betweenthe p-side electrode and the p-type compound semiconductor layer. Toobtain the current confinement structure, a current confinement layerwhich is made of an insulating material (for example, SiO_(X), SiN_(X),or AlO_(X)) may be formed between the p-side electrode and the p-typecompound semiconductor layer, a mesa structure may be formed by etchingthe p-type compound semiconductor layer using an RIE method or the like,a current confinement region may be formed by oxidizing a partial layerof the laminated layers of the p-type compound semiconductor layerpartially in the horizontal direction, a region with reducedconductivity may be formed by performing ion implantation of impuritiesinto the p-type compound semiconductor layer, or a combination of thesemethods may be employed as appropriate. Here, the p-side electrode needsto be electrically connected to a portion of the p-type compoundsemiconductor layer where current flows by current confinement.

For example, the support substrate may be constructed of varioussubstrates such as a GaN substrate, a sapphire substrate, a GaAssubstrate, a SiC substrate, an alumina substrate, a ZnS substrate, a ZnOsubstrate, a LiMgO substrate, a LiGaO₂ substrate, a MgAl₂O₄ substrate,and an InP substrate or may be constructed of an insulating substratemade of AlN or the like, a semiconductor substrate made of Si, SiC, Ge,or the like, or a metal or alloy substrate, and, as the supportsubstrate, it is preferable to use a substrate having conductivity or itis preferable to use a metal or alloy substrate from the viewpoint ofmechanical characteristics, elastic deformation, plastic deformationcharacteristics, heat radiation characteristics, or the like. Thethickness of the support substrate may be exemplified, for example, as0.05 to 0.5 mm. As a method of fixing the second optical reflectivelayer to the support substrate, it is possible to use a known methodsuch as a solder bonding method, a room-temperature bonding method, abonding method using an adhesive tape, or a bonding method using waxbonding and it is desirable that a solder bonding method or aroom-temperature bonding method be employed from the viewpoint ofsecuring a conductivity. For example, in the case where a siliconsemiconductor substrate which is a conductive substrate is used as thesupport substrate, it is desirable that a method in which bonding ispossible at a low temperature equal to or less than 400° C. be employedin order to suppress warping due to the difference in the coefficientsof thermal expansion. In the case where a GaN substrate is used as thesupport substrate, the bonding temperature may be equal to or greaterthan 400° C.

The n-side electrode preferably has a single layer configuration or amultilayer configuration including at least one type of metal (includingalloys) selected from the group consisting of, for example, gold (Au),silver (Ag), palladium (Pd), platinum (Pt), nickel (Ni), Ti (titanium),vanadium (V), tungsten (W), chromium (Cr), aluminum (Al), copper (Cu),zinc (Zn), tin (Sn), and indium (In), which may be specificallyexemplified, for example, by Ti/Au, Ti/Al, Ti/Al/Au, Ti/Pt/Au, Ni/Au,Ni/Au/Pt, Ni/Pt, Pd/Pt, and Ag/Pd. Here, a layer prior to “/” ispositioned closer to the active layer than a layer subsequent to “/” inthe multilayer configuration. This refers to the same in the followingdescription. The n-side electrode may be deposited, for example, using aPVD method such as a vacuum evaporation method or a sputtering method.

A transparent conductive material that constitutes the n-side electrodeor the p-side electrode may be exemplified by indium-tin oxide (ITOincluding Sn-doped In₂O₃, crystalline ITO, and amorphous ITO),indium-zinc oxide (IZO), indium-gallium oxide (IGO), indium-dopedgallium-zinc oxide (IGZO, In—GaZnO₄), IFO (F-doped In₂O₃), tin oxide(SnO₂), ATO (Sb-doped SnO₂), FTO (F-doped SnO₂), zinc oxide (ZnOincluding Al-doped ZnO or B-doped ZnO). As the p-side electrode, it ispossible to employ a transparent conductive film that includes, as abase layer, gallium oxide, titanium oxide, niobium oxide, nickel oxide,or the like. A material which constitutes the p-side electrode is notlimited to a transparent conductive material and, as the material, it ispossible to use a metal such as palladium (Pd), platinum (Pt), nickel(Ni), gold (Au), cobalt (Co), rhodium (Rh), or the like, depending onthe arrangement of the second optical reflective layer and the p-sideelectrode. The p-side electrode may be constructed of at least one ofthese material types. The p-side electrode may be deposited, forexample, using a PVD method such as a vacuum evaporation method or asputtering method.

A pad electrode may be provided on the n-side electrode or the p-sideelectrode to electrically connect the n-side electrode or the p-sideelectrode to an external electrode or circuit. It is desirable that thepad electrode have a single layer configuration or a multilayerconfiguration including at least one type of metal selected from thegroup consisting of titanium (Ti), aluminum (Al), platinum (Pt), gold(Au), nickel (Ni), and palladium (Pd). As an alternative, the padelectrode may have a multilayer configuration which is exemplified by aTi/Pt/Au multilayer configuration, a Ti/Au multilayer configuration, aTi/Pd/Au multilayer configuration, a Ti/Pd/Au multilayer configuration,a Ti/Ni/Au multilayer configuration, and a Ti/Ni/Au/Cr/Au multilayerconfiguration. In the case where the n-side electrode is constructed ofan Ag layer or an Ag/Pd layer, it is preferable that a cover metal layerwhich is made of, for example, Ni/TiW/Pd/TiW/Ni be formed on a surfaceof the n-side electrode and a pad electrode which is made of, forexample, a multilayer configuration of Ti/Ni/Au or a multilayerconfiguration of Ti/Ni/Au/Cr/Au be formed on the cover metal layer.

The optical reflective layer (distributed Bragg reflector (DBR) layer)is constructed of, for example, a semiconductor multilayer film or adielectric multilayer film. The dielectric material may be exemplified,for example, by an oxide such as Si, Mg, Al, Hf, Nb, Zr, Sc, Ta, Ga, Zn,Y, B, or Ti, a nitride (for example, SiN_(X), AlN_(X), AlGaN, GaN_(X),or BN_(X)), a fluoride, or the like. Specifically, the dielectricmaterial may be exemplified by SiO_(X), TiO_(X), NbO_(X), ZrO_(X),TaO_(X), ZnO_(X), AlO_(X), HfO_(X), SiN_(X), AlN_(X), or the like. Theoptical reflective layer may be obtained by alternately laminating twoor more types of dielectric films that are made of dielectric materialswith different refractive indices among those dielectric materials. Itis preferable to employ a dielectric multilayer film such as, forexample, SiO_(X)/SiN_(Y), SiO_(X)/NbO_(Y), SiO_(X)/ZrO_(Y), orSiO_(X)/AlN_(Y). A material which constitutes each dielectric film, thethickness, the number of laminated layers, or the like of eachdielectric film may be selected as appropriate to obtain a desiredoptical reflectance. The thickness of each dielectric film may beappropriately adjusted by the material used or the like and isdetermined by the emission wavelength λ₀ and a refractive index n of thematerial used at the emission wavelength λ₀. Specifically, the thicknessof each dielectric film is preferably an odd multiple of λ₀/(4n). Forexample, in a surface-emitting laser device with an emission wavelengthλ₀ of 450 nm, the thickness may be exemplified as 40 to 70 nm when theoptical reflective layer is constructed of SiO_(X)/NbO_(Y). The numberof laminated layers may be exemplified as 2 or more, preferably about 5to about 20. The total thickness of the optical reflective layer may beexemplified, for example, as about 0.6 to about 1.7 μm.

As an alternative, it is desirable that the first optical reflectivelayer have a dielectric film including at least nitrogen (N) atoms andit is desirable that the dielectric film including nitrogen atoms be thetop layer of the dielectric multilayer film. As an alternative, it isdesirable that the first optical reflective layer be covered with adielectric material layer including at least nitrogen atoms. As analternative, it is desirable that the surface of the first opticalreflective layer be made into a layer including at least nitrogen atoms(hereinafter referred to as a “surface layer”) by performing anitridation process on the surface of the first optical reflectivelayer. It is preferable that the thickness of the dielectric film or thedielectric material layer including at least nitrogen atoms (i.e., thesurface layer) be an odd multiple of λ₀/(4n). Specifically, as amaterial that constitutes the dielectric film or the dielectric materiallayer including at least nitrogen atoms, it is possible to employSiN_(X), SiO_(X)N_(Z), or the like. When a compound semiconductor layerwhich covers the first optical reflective layer is formed by forming adielectric film or a dielectric material layer including at leastnitrogen atoms (i.e., a surface layer), it is possible to improve theoffset between the crystalline axis of the compound semiconductor layerthat covers the first optical reflective layer and the crystalline axisof the GaN substrate and it is possible to increase the quality of thelaminate structural body which becomes a resonator.

The optical reflective layer may be formed on the basis of a well-knownmethod which may be specifically exemplified, for example, by a PVDmethod such as a vacuum evaporation method, a sputtering method, areactive sputtering method, an ECR plasma sputtering method, a magnetronsputtering method, an ion beam assisted evaporation method, an ionplating method, or a laser ablation method; various CVD methods; acoating method such as a spraying method, a spin coating method, or adipping method; a combination of two or more of these methods; acombination of these methods with one or more of overall or partialpretreatment, irradiation with inert gas (Ar, He, Xe or the like) orplasma, irradiation with oxygen or ozone gas or plasma, an oxidationprocess (heat treatment), and an exposure process.

In addition, it is preferable that the substrate be constructed of a GaNsubstrate,

the off-angle of the surface orientation of the surface of the GaNsubstrate be within 0.4°, preferably within 0.40°,

the area of the first optical reflective layer be equal to or less than0.8S₀ when S₀ represents the area of the GaN substrate, and

a thermal expansion mitigation film be formed as a bottom layer of thefirst optical reflective layer on the GaN substrate or the coefficientof linear thermal expansion (CTE) of the bottom layer of the firstoptical reflective layer which is in contact with the GaN substratesatisfy the following:1×10⁻⁶/K≤CTE≤1×10⁻⁶/K,preferably 1×10⁻⁶/K<CTE≤1×10⁻⁶/K.

It is possible to reduce the surface roughness of the p-type compoundsemiconductor layer by predefining the proportion of the area of thefirst optical reflective layer and the off-angle of the surfaceorientation of the crystalline surface of the surface of the GaNsubstrate in this manner. That is, it is possible to form a p-typecompound semiconductor layer with preferable surface morphology.Therefore, it is possible to obtain a second optical reflective layerwith high flatness. That is, it is possible to obtain a desired opticalreflectance and it is difficult for variations in the characteristics tooccur. In addition, by forming a thermal expansion mitigation film orpredefining the CTE value, it is possible to avoid the occurrence of theproblem of separation of the first optical reflective layer from the GaNsubstrate due to the difference in the coefficient of linear thermalexpansion between the GaN substrate and the first optical reflectivelayer, and it is thus possible to provide a highly reliablesurface-emitting laser device. Further, when the GaN substrate is used,it is difficult for dislocations in the compound semiconductor layer tooccur and it is possible to avoid the problem of the increase in thethermal resistance of the surface-emitting laser device. Therefore, itis possible to give high reliability to the surface-emitting laserdevice and it is also possible to provide an n-side electrode at a sideof the GaN substrate (i.e., at a rear surface side of the substrate)different from the side thereof at which the p-side electrode isprovided.

The term “off-angle of the surface orientation of the surface of the GaNsubstrate” refers to an angle formed by the surface orientation of thesurface of the GaN substrate and a line which is normal to the surfaceof the GaN substrate when viewed macroscopically. When S₀ represents thearea of the GaN substrate, it is predefined that the area of the firstoptical reflective layer be equal to or less than 0.8S₀, where the term“area S₀ of the GaN substrate” refers to an area of the GaN substratethat remains when a final surface-emitting laser device has beenobtained. In these cases, the bottom layer of the first opticalreflective layer does not function as an optical reflective layer.

It is possible to provide a form in which the thermal expansionmitigation film is made of at least one type of material selected fromthe group consisting of silicon nitride (SiN_(X)), aluminum oxide(AlO_(X)), niobium oxide (NbO_(X)), tantalum oxide (TaO_(X)), titaniumoxide (TiO_(X)), magnesium oxide (MgO_(X)), zirconium oxide (ZrO_(X)),and aluminum nitride (AlN_(X)). The values of the suffix “X” added tothe chemical formula of each material or suffixes “Y” and “Z” which aredescribed later include not only values based on stoichiometry but alsovalues other than values based on stoichiometry. This refers to the samein the following description. When t₁ represents the thickness of thethermal expansion mitigation film, λ₀ represents the emission wavelengthof the surface-emitting laser device, and n₁ represents the refractiveindex of the thermal expansion mitigation film, it is desirable tosatisfy the following:t ₁=λ₀/(4n ₁),preferably t ₁=λ₀/(λn ₁).

Here, the value of the thickness t₁ of the thermal expansion mitigationfilm may be essentially arbitrary and may be, for example, equal to orless than 1×10⁻⁷ m.

It is possible to provide a form in which the bottom layer of the firstoptical reflective layer is made of at least one type of materialselected from the group consisting of silicon nitride (SiN_(X)),aluminum oxide (AlO_(X)), niobium oxide (NbO_(X)), tantalum oxide(TaO_(X)), titanium oxide (TiO_(X)), magnesium oxide (MgO_(X)),zirconium oxide (ZrO_(X)), and aluminum nitride (AlN_(X)). When t₁represents the thickness of the bottom layer of the first opticalreflective layer, λ₀ represents the emission wavelength of thesurface-emitting laser device, and n₁ represents the refractive index ofthe bottom layer of the first optical reflective layer, it is desirableto satisfy the following:t ₁=λ₀/(4n ₁),preferably t ₁=λ₀/(2n ₁).

Here, the value of the thickness t₁ of the bottom layer of the firstoptical reflective layer may be essentially arbitrary and may be, forexample, equal to or less than 1×10⁻⁷ m.

The surface-emitting laser device of embodiment 5 will now be described.

In the surface-emitting laser device of embodiment 5, the plan shape ofthe first optical reflective layer 51 is a regular hexagon. The regularhexagon is disposed or arranged such that the compound semiconductorlayer is epitaxially grown laterally in the [11-20] direction or in thesame crystalline direction as the [11-20] direction. However, the shapeof the first optical reflective layer 51 is not limited to a regularhexagon but may be, for example, a circle, a grid, or a stripe.

The laminate structural body 20 includes an n-type compoundsemiconductor layer 21, an active layer 23, and a p-type compoundsemiconductor layer 22 which are made of a GaN-based compoundsemiconductor. More specifically, the laminate structural body 20 ismade by laminating:

an n-type compound semiconductor layer 21 which is made of a GaN-basedcompound semiconductor and has a first surface 21 a and a second surface21 b opposite to the first surface 21 a;

an active layer (light emitting layer) 23 which is made of a GaN-basedcompound semiconductor and is in contact with the second surface 21 b ofthe n-type compound semiconductor layer 21; and

a p-type compound semiconductor layer 22 which is made of a GaN-basedcompound semiconductor and has a first surface 22 a that is in contactwith the active layer 23 and a second surface 22 b opposite to the firstsurface 22 a.

The p-side electrode 42 and the second optical reflective layer 52,which is made of a dielectric multilayer film, are formed on the secondsurface 22 b of the p-type compound semiconductor layer 22 and then-side electrode 41 is formed on one surface 11 b of the substrate 11which is opposite to the other surface 11 a of the substrate 11 on whichthe laminate structural body 20 has been formed. The first opticalreflective layer 51 which is made of a dielectric multilayer film isformed on the surface 11 a of the substrate 11 such that the firstoptical reflective layer 51 is in contact with the first surface 21 a ofthe n-type compound semiconductor layer 21.

Here, the surface-emitting laser device of embodiment 5 is made of asurface-emitting laser device that emits light from the top surface ofthe p-type compound semiconductor layer 22 via the second opticalreflective layer 52. Specifically, the surface-emitting laser device ofembodiment 5 is a second optical reflective layer emitting type ofsurface-emitting laser device which emits light from the second surface22 b of the p-type compound semiconductor layer 22 via the secondoptical reflective layer 52. The substrate 11 remains. The configurationand structure of the laminate structural body 20 may be substantiallythe same as those of the laminate structural body 20 in thesurface-emitting laser device described in embodiments 1 to 3.

In the surface-emitting laser device of embodiment 5 or embodiments 6 to10 described later, a current confinement layer 43 which is made of aninsulating material such as SiO_(X), SiN_(X), or AlO_(X) is formedbetween the p-side electrode 42 and the p-type compound semiconductorlayer 22. An opening 43A is formed in the current confinement layer 43and the p-type compound semiconductor layer 22 is exposed at the bottomof the opening 43A. The p-side electrode 42 is formed on the secondsurface 22 b of the p-type compound semiconductor layer 22 over thecurrent confinement layer 43 and the second optical reflective layer 52is formed on the p-side electrode 42. A pad electrode 44 for electricalconnection to an external electrode or circuit is connected to the topof an edge portion of the p-side electrode 42. In the surface-emittinglaser device of embodiment 5 or embodiments 6 and 7 described later, theplan shape of the first optical reflective layer 51 is a regular hexagonand the plan shape of each of the second optical reflective layer 52 andthe opening 43A formed in the current confinement layer 43 is a circle.The first optical reflective layer 51 and the second optical reflectivelayer 52 are each shown as a single layer for simple illustrationalthough each has a multilayer structure. The shape of the currentconfinement layer 43 is not essential.

In the surface-emitting laser device of embodiment 5, the distancebetween the first optical reflective layer 51 and the second opticalreflective layer 52 is equal to or greater than 0.15 μm and equal to orless than 50 μm and is specifically 10 μm, for example. A line which isnormal to the first optical reflective layer 51 and passes through thecenter of gravity of the area of the first optical reflective layer 51is denoted by LN₁ and a line which is normal to the second opticalreflective layer 52 and passes through the center of gravity of the areaof the second optical reflective layer 52 is denoted by LN₂, and LN₁ andLN₂ coincide with each other in the example shown in FIG. 2A.

The n-type compound semiconductor layer 21 is made of an n-type GaNlayer which is 5 μm thick, the active layer 23 has the configuration andstructure described in embodiments 1 to 3, and the p-type compoundsemiconductor layer 22 is configured in two layers, a p-type AlGaNelectron barrier layer (which is 10 nm thick) and a p-type GaN layer.The electron barrier layer is positioned at the side of the activelayer. The n-side electrode 41 is made of Ti/Pt/Au and the p-sideelectrode 42 is made of a transparent dielectric material, specificallyof ITO, the pad electrode 44 is made of Ti/Pd/Au or Ti/Pt/Au, and thefirst optical reflective layer 51 and the second optical reflectivelayer 52 are each made in a laminate structure of SiN_(X) and SiO_(Y)layers (where the total number of laminated layers of the dielectricmultilayer film is 20) with each layer being λ₀/(4n) thick.

In the surface-emitting laser device of embodiment 5, S₁<S₂ is satisfiedwhen Si represents the area of a portion of the first optical reflectivelayer 51 which is in contact with the first surface 21 a of the n-typecompound semiconductor layer 21 (i.e., a portion thereof which isopposite to the second optical reflective layer 52) and S₂ representsthe area of a portion of the second optical reflective layer 52 which isopposite to the second surface 22 b of the p-type compound semiconductorlayer 22 (i.e., a portion thereof which is opposite to the first opticalreflective layer 51).

A method of manufacturing a surface-emitting laser device of embodiment5 will now be described on the basis of FIGS. 3A, 3B, and 3C which areschematic partial sectional views of a substrate or the like.

[Process 500]

A first optical reflective layer 51 is formed on a substrate(specifically, a GaN substrate) 11. Specifically, first, a dielectricmultilayer film is formed on the substrate 11 over the entire surfacethereof on the basis of a sputtering method and is then patterned on thebasis of a photolithography technology and a dry etching technology toobtain a first optical reflective layer 51 (see FIG. 3A).

[Process 510]

Thereafter, an n-type compound semiconductor layer 21, an active layer23, and a p-type compound semiconductor layer 22 are formed over theentire surface. Specifically, an n-type compound semiconductor layer 21which is made of n-type GaN is formed over the entire surface on thebasis of an MOCVD method, which applies epitaxial growth in the lateraldirection, such as an ELO method (using TMG gas and SiH₄ gas).Subsequently, an active layer 23 and a p-type compound semiconductorlayer 22 are formed over the entire surface. Specifically, an activelayer 23 is formed on the n-type compound semiconductor layer 21 usingTMG gas and TMI gas on the basis of an epitaxial growth method, andthereafter an electron barrier layer is formed using TMG gas, TMA gas,and Cp₂Mg gas and a p-type GaN layer is formed using TMG gas and Cp₂Mggas to obtain a p-type compound semiconductor layer 22. The laminatestructural body 20 can be obtained through the above processes. That is,the laminate structural body 20 is epitaxially grown on the substrate(specifically, GaN substrate) 11 including the first optical reflectivelayer 51, the laminate structural body 20 being made by laminating: ann-type compound semiconductor layer 21 which is made of a GaN-basedcompound semiconductor and has a first surface 21 a and a second surface21 b opposite to the first surface 21 a;

an active layer 23 which is made of a GaN-based compound semiconductorand is in contact with the second surface 21 b of the n-type compoundsemiconductor layer 21; and a p-type compound semiconductor layer 22which is made of a GaN-based compound semiconductor and has a firstsurface 22 a that is in contact with the active layer 23 and a secondsurface 22 b opposite to the first surface 22 a.

The structure shown in FIG. 3B can be obtained in this manner.

[Process 520]

Then, a current confinement layer 43, which is made of a 0.2 μm thickinsulating material and has an opening 43A, is formed on the secondsurface 22 b of the p-type compound semiconductor layer 22 on the basisof a well-known method.

[Process 530]

Thereafter, a p-side electrode and a second optical reflective layerwhich are opposite to the first optical reflective layer 51 are formedon the p-type compound semiconductor layer 22. Specifically, a p-sideelectrode 42 and a second optical reflective layer 52 which is made of adielectric multilayer film are formed on the second surface 22 b of thep-type compound semiconductor layer 22. More specifically, a p-sideelectrode 42 which is made of a 50 nm thick ITO layer is formed on thesecond surface 22 b of the p-type compound semiconductor layer 22 overthe current confinement layer 43, for example, on the basis of a liftoffmethod and a pad electrode 44 is formed on the p-side electrode 42 overthe current confinement layer 43 on the basis of a well-known method.The structure shown in FIG. 3C can be obtained in this manner.Thereafter, a second optical reflective layer 52 is formed on the p-sideelectrode 42 over the pad electrode 44 on the basis of a well-knownmethod. On the other hand, an n-side electrode 41 is formed on the othersurface 11 b of the substrate 11 on the basis of a well-known method. Asurface-emitting laser device of embodiment 5 having the structure shownin FIG. 2A can be obtained in this manner.

[Process 540]

Thereafter, the surface-emitting laser device is separated by performingso-called device separation and the side surface or the exposed surfaceof the laminate structural body 20 is covered, for example, with aninsulating film which is made of SiOx. A terminal or the like is formedon the basis of a well-known method to connect the n-side electrode 41or the pad electrode 44 to an external circuit or the like and packagingor sealing is performed to complete a surface-emitting laser device ofembodiment 5.

As described above, when the n-type compound semiconductor layer 21 isformed on the substrate 11 including the first optical reflective layer51 formed thereon through lateral growth on the basis of a method ofepitaxial growth in the lateral direction such as an epitaxial lateralovergrowth (ELO) method, a lot of crystalline defects may occur at aportion of the first optical reflective layer 51 where regions of then-type compound semiconductor layer 21 meet as the n-type compoundsemiconductor layer 21 is epitaxially grown from edge portions of thefirst optical reflective layer 51 toward a central portion of the firstoptical reflective layer 51.

In a surface-emitting laser device of a modified example of embodiment5, as shown in FIG. 2B, the center of gravity of the area of a secondoptical reflective layer 52 is not on a line LN₁ normal to a firstoptical reflective layer 51 which passes through the center of gravityof the area of the first optical reflective layer 51. A line LN₂ normalto the second optical reflective layer 52 which passes through thecenter of gravity of the area of the second optical reflective layer 52coincides with a line normal to the active layer 23 which passes throughthe center of gravity of the area of the active layer 23 (specifically,the center of gravity of an area of the active layer 23 whichconstitutes a device region). In other words, the center of gravity ofthe area of the active layer 23 is not on the line LN₁ normal to thefirst optical reflective layer 51 which passes through the center ofgravity of the area of the first optical reflective layer 51. Thisprevents the meeting portion having a lot of crystalline defects(specifically, a portion positioned at or near the normal line LN₁) frombeing positioned at a central portion of the device region and preventsor reduces adverse effects on the characteristics of thesurface-emitting laser device. In addition, S₃<S₄ is satisfied when S₃represents the area of a portion of the first optical reflective layer51, which is in contact with the first surface 21 a of the n-typecompound semiconductor layer 21 (i.e., a portion thereof which isopposite to the second optical reflective layer 52) and constitutes thedevice region, and S₄ represents the area of a portion of the secondoptical reflective layer 52 which is opposite to the second surface 22 bof the p-type compound semiconductor layer 22 (i.e., a portion thereofwhich is opposite to the first optical reflective layer 51) andconstitutes the device region.

Embodiment 6

Embodiment 6 is a modification of embodiment 5. In a surface-emittinglaser device of embodiment 6, light generated from an active layer 23 isemitted outside from the top surface of an n-type compound semiconductorlayer 21 via a first optical reflective layer 51 as shown in FIG. 4Awhich is a schematic partial sectional view. That is, thesurface-emitting laser device of embodiment 6 is a first opticalreflective layer emitting type of surface-emitting laser device. In thesurface-emitting laser device of embodiment 6, a second opticalreflective layer 52 is fixed to a support substrate 46, which is made ofa silicon semiconductor substrate, on the basis of solder bonding via abonding layer 45 which is made of a solder layer including tin (Sn) or agold (Au) layer.

In embodiment 6, an active layer 23, a p-type compound semiconductorlayer 22, a p-side electrode 42, and a second optical reflective layer52 are sequentially formed on the n-type compound semiconductor layer 21and then the second optical reflective layer 52 is fixed to the supportsubstrate 46. Thereafter, the substrate 11 is removed using the firstoptical reflective layer 51 as a polishing stopper layer to expose then-type compound semiconductor layer 21 (i.e., a first surface 21 a ofthe n-type compound semiconductor layer 21) and the first opticalreflective layer 51. An n-side electrode 41 is then formed on the n-typecompound semiconductor layer 21 (i.e., on the first surface 21 a of then-type compound semiconductor layer 21).

The distance between the first optical reflective layer 51 and thesecond optical reflective layer 52 is equal to or greater than 0.15 μmand equal to or less than 50 μm and is specifically 10 μm, for example.In the surface-emitting laser device of embodiment 6, the first opticalreflective layer 51 and the n-side electrode 41 are spaced apart fromeach other. That is, the first optical reflective layer 51 and then-side electrode 41 have an offset therebetween. The spacing distance iswithin 1 mm and is specifically 0.05 mm on average, for example.

A method of manufacturing a surface-emitting laser device of embodiment6 will now be described with reference to FIGS. 5A and 5B which areschematic partial sectional views of a laminate structural body or thelike.

[Process 600]

First, a structure shown in FIG. 2A is obtained by performing the sameprocesses as [Process 500] to [Process 530] of embodiment 5. However,the n-side electrode 41 is not formed.

[Process 610]

Thereafter, a second optical reflective layer 52 is fixed to a supportsubstrate 46 via a bonding layer 45. The structure shown in FIG. 5A canbe obtained in this manner.

[Process 620]

Then, the substrate (GaN substrate) 11 is removed to expose the firstsurface 21 a of the n-type compound semiconductor layer 21 and the firstoptical reflective layer 51. Specifically, first, the thickness of thesubstrate 11 is reduced on the basis of a mechanical polishing methodand then a remaining portion of the substrate 11 is removed on the basisof a CMP method. In this manner, the first surface 21 a of the n-typecompound semiconductor layer 21 and the first optical reflective layer51 are exposed to obtain a structure shown in FIG. 9B.

[Process 630]

Thereafter, an n-side electrode 41 is formed on the first surface 21 aof the n-type compound semiconductor layer 21 on the basis of awell-known method. A surface-emitting laser device of embodiment 6having the structure shown in FIG. 4A can be obtained in this manner.

[Process 640]

Thereafter, the surface-emitting laser device is separated by performingso-called device separation and the side surface or the exposed surfaceof the laminate structural body 20 is covered, for example, with aninsulating film which is made of SiOx. A terminal or the like is formedon the basis of a well-known method to connect the n-side electrode 41or the pad electrode 44 to an external circuit or the like and packagingor sealing is performed to complete a surface-emitting laser device ofembodiment 6.

In the method of manufacturing the surface-emitting laser device ofembodiment 6, the substrate on which the first optical reflective layerhas been formed is removed. Therefore, the first optical reflectivelayer functions as a polishing stopper layer when the substrate isremoved. As a result, it is possible to suppress the occurrence ofvariations in the removal of the substrate within the surface thereofand to suppress the occurrence of variations in the thickness of then-type compound semiconductor layer and also to achieve uniformity ofthe length of the resonator. Therefore, it is possible to achievestabilization of the characteristics of the obtained surface-emittinglaser device. In addition, since the surface (flat surface) of then-type compound semiconductor layer at the interface between the n-typecompound semiconductor layer and the first optical reflective layer isflat, light scattering at the flat surface may be minimized as much aspossible.

In the example of the surface-emitting laser device shown in FIG. 4A, anend portion of the n-side electrode 41 is spaced apart from the firstoptical reflective layer 51. On the other hand, in the example of thesurface-emitting laser device shown in FIG. 4B, an end portion of then-side electrode 41 extends to an outer edge of the first opticalreflective layer 51. Alternatively, an n-side electrode may be formedsuch that an end portion of the n-side electrode is in contact with thefirst optical reflective layer.

Embodiment 7

Embodiment 7 is a modification of embodiments 5 and 6. FIG. 6 shows aschematic partial sectional view of a surface-emitting laser device ofembodiment 7. In the surface-emitting laser device of embodiment 7, theoff-angle of the surface orientation of a crystalline surface of asurface 11 a of a GaN substrate 11 is within 0.4°, preferably within0.40°, and the area of the first optical reflective layer 51 is equal toor less than 0.8S₀ when S₀ represents the area of the GaN substrate 11.The lower limit of the area of the first optical reflective layer 51 maybe exemplified as, but is not limited to, 0.004×S₀. A thermal expansionmitigation film 53 is formed as a bottom layer of the first opticalreflective layer 51 on the GaN substrate 11 or the coefficient of linearthermal expansion (CTE) of the bottom layer (corresponding to thethermal expansion mitigation film 53) of the first optical reflectivelayer 51 which is in contact with the GaN substrate 11 satisfies thefollowing:1×10⁻⁶/K≤CTE≤1×10⁻⁶/K,preferably, 1×10⁻⁶/K<CTE≤1×10⁻⁵/K.

Specifically, the thermal expansion mitigation film 53 (the bottom layerof the first optical reflective layer 51) is made, for example, ofsilicon nitride (SiN_(X)) that satisfies:t ₁=λ₀/(2n ₁).

The thermal expansion mitigation film 53 (the bottom layer of the firstoptical reflective layer 51) having such a film thickness is transparentto light of a wavelength λ₀ and does not function as an opticalreflective layer. The CTE values of the GaN substrate 11 and siliconnitride (SiN_(X)) are as in Table 6. The CTE values are values at 25° C.

TABLE 6 GaN substrate: 5.59 × 10⁻⁶/K Silicon nitride (SiNx): 2.6-3.5 ×10⁻⁶/K

To manufacture the surface-emitting laser device of embodiment 7, first,a thermal expansion mitigation film 53 which constitutes the bottomlayer of a first optical reflective layer 51 is formed and a remainingportion of the first optical reflective layer 51 which is made of adielectric multilayer film is formed on the thermal expansion mitigationfilm 53. Patterning is then performed to obtain the first opticalreflective layer 51. Thereafter, the same processes as [Process 510] to[Process 540] of embodiment 5 may be performed.

In embodiment 7, the relationship between the off-angle and the surfaceroughness Ra of the p-type compound semiconductor layer 22 has beenexamined. The results are shown in Table 7 below. From Table 7, it isseen that the value of surface roughness Ra of the p-type compoundsemiconductor layer 22 is high when the off-angle has exceeded 0.4°.That is, by making the off-angle equal to or less than 0.4°, preferablywithin 0.40°, it is possible to suppress step bunching during growth ofthe compound semiconductor layer and it is possible to reduce the valueof the surface roughness Ra of the p-type compound semiconductor layer22. As a result, a second optical reflective layer 52 with high flatnesscan be obtained and it is difficult for variations in thecharacteristics such as optical reflectance to occur.

TABLE 7 Off-angle (degrees) Surface roughness Ra (nm) 0.35 0.87 0.380.95 0.43 1.32 0.45 1.55 0.50 2.30

In addition, the relationship between the area S₀ of the GaN substrate11, the area of the first optical reflective layer 51, and the surfaceroughness Ra of the p-type compound semiconductor layer 22 has beenexamined. The results are shown in Table 8 below. It is seen from Table8 that the value of the surface roughness Ra of the p-type compoundsemiconductor layer 22 can be lowered by making the area of the firstoptical reflective layer 51 equal to or less than 0.8S₀.

TABLE 8 Area of first optical reflective layer 51 Surface roughness Ra(nm) 0.88S₀ 1.12 0.83S₀ 1.05 0.75S₀ 0.97 0.69S₀ 0.91 0.63S₀ 0.85

From the above results, it is seen that the surface roughness Ra of thep-type compound semiconductor layer 22 (the second surface 22 b of thep-type compound semiconductor layer 22) is preferably equal to or lessthan 1.0 nm.

In addition, manufacturing a surface-emitting laser device having thesame configuration and structure as embodiment 7 without forming thethermal expansion mitigation film 53, but with a bottom layer of thefirst optical reflective layer 51 made of SiOx (CTE: 0.51−0.58×10⁻⁶/K),may cause separation of the first optical reflective layer 51 from theGaN substrate 11 during deposition of the laminate structural body 20depending on the manufacturing conditions. On the other hand, inembodiment 7, separation of the first optical reflective layer 51 fromthe GaN substrate 11 does not occur during deposition of the laminatestructural body 20.

In the surface-emitting laser device of embodiment 7, it is possible toreduce the surface roughness of the p-type compound semiconductor layersince the off-angle of the surface orientation of the crystallinesurface of the surface of the GaN substrate and the proportion of thearea of the first optical reflective layer are predefined as describedabove. That is, it is possible to form a p-type compound semiconductorlayer with preferable surface morphology. As a result, it is possible toobtain a second optical reflective layer with high flatness andtherefore a desired optical reflectance can be obtained and it isdifficult for variations in the characteristics of the surface-emittinglaser device to occur. In addition, since a thermal expansion mitigationfilm is formed or the CTE values are predefined, it is possible to avoidthe occurrence of the problem of separation of the first opticalreflective layer from the GaN substrate due to the difference in thecoefficient of linear thermal expansion between the GaN substrate andthe first optical reflective layer, and it is thus possible to provide ahighly reliable surface-emitting laser device. Further, since the GaNsubstrate is used, it is difficult for dislocations in the compoundsemiconductor layer to occur and it is possible to avoid the problem ofthe increase in the thermal resistance of the surface-emitting laserdevice. Therefore, it is possible to give high reliability to thesurface-emitting laser device and it is also possible to provide ann-side electrode at a side of the GaN substrate (i.e., at a rear surfaceside of the substrate) different from the side thereof at which thep-side electrode is provided.

Embodiment 8

Embodiment 8 is a modification of embodiment 6. When the thickness ofthe n-type compound semiconductor layer 21 is large, light may bescattered out of the resonator to be lost when bouncing between thefirst optical reflective layer 51 and the second optical reflectivelayer 52, which may cause an increase in the threshold of thesurface-emitting laser device and a reduction in the differentialefficiency and may further cause problems such as an increase in theoperating voltage and a reduction in the reliability. However, reducingthe thickness of the n-type compound semiconductor layer 21 through apolishing method often involves difficulties such as wafer cracks andnon-uniformity in the resonator length.

In a surface-emitting laser device of embodiment 8, as shown in FIG. 7Awhich is a schematic partial sectional view thereof, a convex portion 21c is formed on a first surface 21 a of the n-type compound semiconductorlayer 21, a first optical reflective layer 51 is formed on the convexportion 21 c, and an n-side electrode 41 is formed on a concave portion21 e surrounding the convex portion 21 c formed on the first surface 21a of the n-type compound semiconductor layer 21. That is, in embodiment8, the n-type compound semiconductor layer 21 has a so-called mesashape. The plan shape of the convex portion 21 c is a circle. By makingthe n-type compound semiconductor layer 21 in a mesa shape in thismanner, it is possible to prevent light from being scattered out of theresonator when bouncing between the first optical reflective layer 51and the second optical reflective layer 52 and to eliminate thepossibility of the occurrence of problems such as an increase in theoperating voltage and a reduction in the reliability.

The plan shape of the n-side electrode 41 is annular (i.e.,ring-shaped). The plan shape of the device region is a circle, and theplan shapes of the first optical reflective layer 51, the second opticalreflective layer 52, and the opening 43A formed in the currentconfinement layer 43 are also a circle.

The height of the convex portion 21 c is less than the thickness of then-type compound semiconductor layer 21 and may be equal to or greaterthan 1×10⁻⁸ m and equal to less than 1×10⁻⁵ m and may be specificallyexemplified, for example, as 2×10⁻⁶ m. The size of the convex portion 21c is greater than that of the first optical reflective layer 51 and isalso greater than that of the device region.

A dielectric layer 28 made of SiO₂, SiN, AlN, ZrO₂, Ta₂O₅, or the likeis formed on a side surface (side wall) 21 d of the convex portion 21 c,which can more reliably prevent light from being scattered out of theresonator when bouncing between the first optical reflective layer 51and the second optical reflective layer 52. It is preferable that therefractive index value of the material that constitutes the dielectriclayer 28 be smaller than the average refractive index value of thematerial that constitutes the n-type compound semiconductor layer 21.

The configuration and structure of the surface-emitting laser device ofembodiment 8 may be the same as those of the surface-emitting laserdevice of embodiment 6, except for the points described above, andtherefore a detailed description thereof is omitted.

A method of manufacturing a surface-emitting laser device of embodiment8 will now be described with reference to FIGS. 8A, 8B, 8C, 9A, 9B, and10 which are schematic partial sectional views of a laminate structuralbody or the like. In the method of manufacturing a surface-emittinglaser device of embodiment 8, first, a laminate structural body 20 isformed on a substrate 11, unlike the method of manufacturing asurface-emitting laser device of embodiment 6. The first opticalreflective layer 51 is formed in subsequent processes.

[Process 800]

First, a laminate structural body 20 made by laminating an n-typecompound semiconductor layer 21 which is made of a GaN-based compoundsemiconductor and has a first surface 21 a and a second surface 21 bopposite to the first surface 21 a, an active layer 23 which is made ofa GaN-based compound semiconductor and is in contact with the secondsurface 21 b of the n-type compound semiconductor layer 21, and a p-typecompound semiconductor layer 22 which is made of a GaN-based compoundsemiconductor and has a first surface 22 a that is in contact with theactive layer 23 and a second surface 22 b opposite to the first surface22 a is formed on a substrate 11 which is constructed of a GaN substrateon the basis of a well-known MOCVD method. Then, a current confinementlayer 43 having an opening 43A is formed on the p-type compoundsemiconductor layer 22 on the basis of a well-known method. Thestructure shown in FIG. 8A can be obtained in this manner.

[Process 810]

Then, a p-side electrode 42 and a second optical reflective layer 52that is made of a dielectric multilayer film are formed on the secondsurface 22 b of the p-type compound semiconductor layer 22.Specifically, a p-side electrode 42 is formed on the second surface 22 bof the p-type compound semiconductor layer 22 over the currentconfinement layer 43, for example, on the basis of a liftoff method anda pad electrode 44 is formed on the p-side electrode 42 over the currentconfinement layer 43 on the basis of a well-known method. The structureshown in FIG. 8B can be obtained in this manner. Thereafter, a secondoptical reflective layer 52 is formed on the p-side electrode 42 overthe pad electrode 44 on the basis of a well-known method. The structureshown in FIG. 8C can be obtained in this manner.

[Process 820]

Thereafter, the second optical reflective layer 52 is fixed to a supportsubstrate 46 via a bonding layer 45. The structure shown in FIG. 9A canbe obtained in this manner.

[Process 830]

Then, the substrate 11 is removed to expose the first surface 21 a ofthe n-type compound semiconductor layer 21. Specifically, first, thethickness of the substrate 11 is reduced on the basis of a mechanicalpolishing method and then a remaining portion of the substrate 11 isremoved on the basis of a CMP method. In addition, the exposed n-typecompound semiconductor layer 21 is partially etched in the thicknessdirection and mirror finishing is performed on the first surface 21 a ofthe n-type compound semiconductor layer 21. The structure shown in FIG.9B can be obtained in this manner.

[Process 840]

Thereafter, a convex portion 21 c and a concave portion 21 e are formedon the first surface 21 a of the n-type compound semiconductor layer 21,a first optical reflective layer 51 that is made of a dielectricmultilayer film is formed on the convex portion 21 c, an n-sideelectrode 41 is formed on the convex portion 21 c and the concaveportion 21 e, and a dielectric layer 28 is formed on a side surface(side wall) 21 d of the convex portion 21 c.

Specifically, an etching resist layer is formed on the n-type compoundsemiconductor layer 21 in a region on which the convex portion 21 c isto be formed on the basis of a well-known method and then an exposedregion of the n-type compound semiconductor layer 21 is etched on thebasis of an RIE method to form the convex portion 21 c and the concaveportion 21 e. The structure shown in FIG. 10 can be obtained in thismanner. Then, a dielectric layer 28 is formed on a side surface (sidewall) 21 d of the convex portion 21 c using a well-known method.

Then, a first optical reflective layer 51 is formed on the convexportion 21 c of the n-type compound semiconductor layer 21 on the basisof a well-known method. Thereafter, an n-side electrode 41 is formed onthe concave portion 21 e of the n-type compound semiconductor layer 21on the basis of a well-known method. A surface-emitting laser device ofembodiment 8 having the structure shown in FIGS. 7A and 7B can beobtained in this manner.

The order in which the convex portion 21 c of the first surface 21 a ofthe n-type compound semiconductor layer 21, the dielectric layer 28, thefirst optical reflective layer 51, and the n-side electrode 41 areformed is not limited to the order described above. For example, thefirst optical reflective layer 51, the convex portion 21 c of the firstsurface 21 a of the n-type compound semiconductor layer 21, thedielectric layer 28, and the n-side electrode 41 may be formed in thisorder and the convex portion 21 c of the first surface 21 a of then-type compound semiconductor layer 21, the dielectric layer 28, then-side electrode 41, and the first optical reflective layer 51 may beformed in this order. Essentially, the convex portion 21 c of the firstsurface 21 a of the n-type compound semiconductor layer 21, thedielectric layer 28, the first optical reflective layer 51, and then-side electrode 41 may be formed appropriately in any order.

[Process 850]

Thereafter, the surface-emitting laser device is separated by performingso-called device separation and the side surface or the exposed surfaceof the laminate structural body is covered, for example, with aninsulating film which is made of SiO₂ or the like. A terminal or thelike is formed on the basis of a well-known method to connect the n-sideelectrode 41 or the pad electrode 44 to an external circuit or the likeand packaging or sealing is performed to complete a surface-emittinglaser device of embodiment 8.

Embodiment 9

Embodiment 9 is a modification of embodiment 8. As shown in FIG. 7Bwhich is a schematic partial sectional view, an annular groove portion21 f is formed around a first optical reflective layer 51 formed on afirst surface 21 a of the n-type compound semiconductor layer 21 and thegroove portion 21 f is filled with an insulating material. That is, aninsulating material layer 29 made of SiO₂, SiN, AlN, ZrO₂, Ta₂O₅, or thelike is formed in the groove portion 21 f. By making the n-type compoundsemiconductor layer 21 in a mesa shape in this manner, i.e., by fillingthe annular groove portion 21 f with an insulating material, it ispossible to prevent light from being scattered out of the resonator whenbouncing between the first optical reflective layer 51 and the secondoptical reflective layer 52 and to eliminate the possibility of theoccurrence of problems such as an increase in the operating voltage anda reduction in the reliability.

The depth of the groove portion 21 f is less than the thickness of then-type compound semiconductor layer 21 and may be equal to or greaterthan 1×10⁻⁸ m and equal to less than 1×10⁻⁵ m and may be specificallyexemplified, for example, as 2×10⁻⁸ m. The inner diameter of the grooveportion 21 f is greater than the first optical reflective layer 51 andis also greater than that of the device region.

The configuration and structure of the surface-emitting laser device ofembodiment 9 may be the same as those of the surface-emitting laserdevice of embodiment 6, except for the points described above, andtherefore a detailed description thereof is omitted.

In the same process of the surface-emitting laser device of embodiment 9as [Process 840] of the surface-emitting laser device of embodiment 8, agroove portion 21 f may be formed instead of the convex portion 21 c onthe first surface 21 a of the n-type compound semiconductor layer 21 andan insulating material layer 29 may be formed in the groove portion 21f.

Alternatively, in the same process as [Process 500] of embodiment 5, agroove portion 21 f is formed on a substrate (specifically, a GaNsubstrate) 11 and an insulating material layer 29 is formed in thegroove portion 21 f, and then a first optical reflective layer 51 isformed on the substrate 11. In the same process as [Process 510] ofembodiment 5, an n-type compound semiconductor layer 21, an active layer23, and a p-type compound semiconductor layer 22 are formed over theentire surface, i.e., over the substrate 11, the first opticalreflective layer 51, and the insulating material layer 29 filled intothe groove portion 21 f, and thereafter the same processes as [Process520] to [Process 540] of embodiment 5 may be performed. In this case, inthe same process as [Process 830] of embodiment 8, the insulatingmaterial layer 29 functions as a polishing stopper layer when thesubstrate 11 has been removed to expose the first surface 21 a of then-type compound semiconductor layer 21 and therefore it is possible toprevent the occurrence of variations of the n-type compoundsemiconductor layer 21 in the thickness direction thereof.

Embodiment 10

Embodiment 10 is also a modification of embodiments 8 and 9. As shown inFIGS. 11A and 11B which are schematic partial sectional views, a lineLN₁ normal to the first optical reflective layer which passes throughthe center of gravity of the area of the first optical reflective layerdoes not coincide with a line LN₂ normal to the second opticalreflective layer which passes through the center of gravity of the areaof a portion of the second optical reflective layer which is opposite tothe p-type compound semiconductor layer. In other words, the center ofgravity of the area of the active layer (specifically, the center ofgravity of an area of the active layer that constitutes the deviceregion) is not on the line LN₁ normal to the first optical reflectivelayer which passes through the center of gravity of the area of thefirst optical reflective layer. The surface-emitting laser device shownin FIG. 11A is a modification of the surface-emitting laser device ofembodiment 8 shown in FIG. 7A and the surface-emitting laser deviceshown in FIG. 11B is a modification of the surface-emitting laser deviceof embodiment 9 shown in FIG. 7B.

In the surface-emitting laser device, the mode where the optical fieldintensity at the center of the resonator is maximized (i.e., thefundamental mode) is often the most stable. In the surface-emittinglaser device of embodiment 10, the line LN₁ normal to the first opticalreflective layer 51 which passes through the center of gravity of thearea of the first optical reflective layer 51 does not coincide with theline LN₂ normal to the second optical reflective layer 52 which passesthrough the center of gravity of the area of a portion of the secondoptical reflective layer 52 which is opposite to the p-type compoundsemiconductor layer 22, or the center of gravity of the area of theactive layer 23 is not on the line LN₁ normal to the first opticalreflective layer 51 which passes through the center of gravity of thearea of the first optical reflective layer 51. In other words, thecentral axis of the mesa shape which serves as a waveguide of the n-typecompound semiconductor layer 21 and the central axis of the deviceregion (i.e., the current injection region) are intentionally displacedfrom each other. Therefore, it is possible to reduce the optical fieldintensity at the central axis of the resonator, thereby lowering thestability of the fundamental mode. Thereby, it is possible to reduce thestability of the fundamental mode during a high power operation and thusto cause kinking, thereby reducing the upper limit of the optical outputof the surface-emitting laser device. Accordingly, this configuration ispreferably employed when it is used in applications where it isdesirable that the upper limit of the output be limited such as, forexample, laser irradiation on a biological body. The offset between thenormal line LN₁ and the normal line LN₂ may be exemplified as0.01-0.251=10 when R₀ represents the diameter of a circle which isassumed to be the plan shape of the device region.

The configuration and structure of the surface-emitting laser device ofembodiment 10 may be the same as those of the surface-emitting laserdevice of embodiments 8 and 9, except for the points described above,and therefore a detailed description thereof is omitted.

Although the present disclosure has been described on the basis ofpreferred embodiments, the present disclosure is not limited theseembodiments. The configurations and structures of the light emittingdevice described above in the embodiments are exemplary and may bechanged as appropriate and the method of manufacturing the lightemitting device of the embodiments may also be changed as appropriate.

Additionally, the present technology may also be configured as below.

[A01]<<Optical Semiconductor Device: First Aspect>>

An optical semiconductor device including a laminate structural body inwhich an n-type compound semiconductor layer, an active layer, and ap-type compound semiconductor layer are laminated in this order, inwhich the active layer includes a multiquantum well structure includinga tunnel barrier layer, and a compositional variation of a well layeradjacent to the p-type compound semiconductor layer is greater than acompositional variation of another well layer.

[A02]

The optical semiconductor device according to [A01], in which band gapenergy of the well layer adjacent to the p-type compound semiconductorlayer is smaller than band gap energy of the other well layer.

[A03]

The optical semiconductor device according to [A01], in which athickness of the well layer adjacent to the p-type compoundsemiconductor layer is greater than a thickness of the other well layer.

[A04]

The optical semiconductor device according to [A03], in which band gapenergy of the well layer adjacent to the p-type compound semiconductorlayer is smaller than band gap energy of the other well layer.

[A05]

The optical semiconductor device according to any one of [A01] to [A04],in which the tunnel barrier layer is formed between a well layer and abarrier layer.

[B01]<< Optical Semiconductor Device: Second Aspect>>

An optical semiconductor device including a laminate structural body inwhich an n-type compound semiconductor layer, an active layer, and ap-type compound semiconductor layer are laminated in this order,

in which the active layer includes a multiquantum well structureincluding a tunnel barrier layer, and

a band gap energy of a well layer adjacent to the p-type compoundsemiconductor layer is smaller than a band gap energy of another welllayer.

[B02]

The optical semiconductor device according to [B01], in which athickness of the well layer adjacent to the p-type compoundsemiconductor layer is greater than a thickness of the other well layer.

[B03]

The optical semiconductor device according to [B01] or [B02], in whichthe tunnel barrier layer is formed between a well layer and a barrierlayer.

[C01] << Optical Semiconductor Device: Third Aspect>>

An optical semiconductor device including a laminate structural body inwhich an n-type compound semiconductor layer, an active layer, and ap-type compound semiconductor layer are laminated in this order,

in which the active layer includes a multiquantum well structureincluding a tunnel barrier layer, and

a thickness of a well layer adjacent to the p-type compoundsemiconductor layer is greater than a thickness of another well layer.

[C02]

The optical semiconductor device according to [C01], in which the tunnelbarrier layer is formed between a well layer and a barrier layer.

[D01]

The optical semiconductor device according to any one of [A01] to [C02],in which a thickness of the tunnel barrier layer is equal to or lessthan 4 nm.

[D02]

The optical semiconductor device according to any one of [A01] to [D01],in which the active layer is made of an AlInGaN-based compoundsemiconductor.

[D03]

The optical semiconductor device according to [D02], in which the tunnelbarrier layer is made of GaN.

[D04]

The optical semiconductor device according to [D02] or [D03], in whichthe n-type compound semiconductor layer is formed on a c-surface of aGaN substrate.

[D05]

The optical semiconductor device according to any one of [D02] to [D04],in which an emission wavelength is equal to or greater than 440 nm.

[E01]

The optical semiconductor device according to [D03] or [D04], in whichthe optical semiconductor device is made of a surface-emitting laserdevice, the off-angle of a surface orientation of a surface of the GaNsubstrate is within 0.4°, preferably within 0.40°,

the area of a first optical reflective layer is equal to or less than0.8S₀ when S₀ represents the area of the GaN substrate, and

a thermal expansion mitigation film is formed as a bottom layer of thefirst optical reflective layer on the GaN substrate.

[E02]

The optical semiconductor device according to [E01], in which thethermal expansion mitigation film is made of at least one type ofmaterial selected from the group consisting of silicon nitride, aluminumoxide, niobium oxide, tantalum oxide, titanium oxide, magnesium oxide,zirconium oxide, and aluminum nitride.

[E03]

The optical semiconductor device according to [E01] or [E02], in whicht₁=Δ₀/(2n₁) is satisfied when t₁ represents the thickness of the thermalexpansion mitigation film, λ₀ represents the emission wavelength of theoptical semiconductor device, and n₁ represents the refractive index ofthe thermal expansion mitigation film.

[E04]

The optical semiconductor device according to [D03] or [D04], in whichthe optical semiconductor device is made of a surface-emitting laserdevice, the off-angle of a surface orientation of a surface of the GaNsubstrate is within 0.4°, preferably within 0.40°,

the area of a first optical reflective layer is equal to or less than0.8S₀ when S₀ represents the area of the GaN substrate, and

the coefficient of linear thermal expansion (CTE) of a bottom layer ofthe first optical reflective layer which is in contact with the GaNsubstrate satisfies:1×10⁻⁶/K≤CTE≤1×10⁻⁵/K,preferably, 1×10⁻⁶/K<CTE≤1×10⁻⁵/K.[E05]

The optical semiconductor device according to [E04], in which the bottomlayer of the first optical reflective layer is made of at least one typeof material selected from the group consisting of silicon nitride,aluminum oxide, niobium oxide, tantalum oxide, titanium oxide, magnesiumoxide, zirconium oxide, and aluminum nitride.

[E06]

The optical semiconductor device according to [E04] or [E05], in whicht₁=Δ₀/(2n₁) is satisfied when t₁ represents the thickness of the bottomlayer of the first optical reflective layer, λ₀ represents the emissionwavelength of the optical semiconductor device, and n₁ represents therefractive index of the bottom layer of the first optical reflectivelayer.

[E07]

The optical semiconductor device according to any one of [E01] to [E06],in which a surface roughness Ra of the p-type compound semiconductorlayer is equal to or less than 1.0 nm.

[F01]

The optical semiconductor device according to any one of [D02] to [E07],in which the optical semiconductor device is made of a surface-emittinglaser device, and a convex portion is formed on a first surface of then-type compound semiconductor layer which is opposite to the activelayer, the first optical reflective layer is formed on the convexportion, and an n-side electrode is formed on a concave portionsurrounding the convex portion formed on the first surface of the n-typecompound semiconductor layer.

[F02]

The optical semiconductor device according to [F01], in which adielectric layer is formed on a side surface of the convex portion.

[F03]

The optical semiconductor device according to [F02], in which therefractive index value of a material that constitutes the dielectriclayer is smaller than the average refractive index value of a materialthat constitutes the n-type compound semiconductor layer.

[F04]

The optical semiconductor device according to any one of [D02] to [E07],in which the optical semiconductor device is made of a surface-emittinglaser device, the first optical reflective layer is formed on a firstsurface of the n-type compound semiconductor layer which is opposite tothe active layer, a groove portion is formed on the first surface of then-type compound semiconductor layer such that the groove portionsurrounds the first optical reflective layer, and the groove portion isfilled with an insulating material.

[F05]

The optical semiconductor device according to any one of [D02] to [F04],in which the optical semiconductor device is made of a surface-emittinglaser device, and a line normal to the first optical reflective layerwhich passes through a center of gravity of the area of the firstoptical reflective layer does not coincide with a line normal to thesecond optical reflective layer which passes through a center of gravityof the area of a portion of the second optical reflective layer which isopposite to the p-type compound semiconductor layer.

[F06]

The optical semiconductor device according to any one of [D02] to [F04],in which the optical semiconductor device is made of a surface-emittinglaser device, and a center of gravity of the area of the active layer isnot on a line normal to the first optical reflective layer which passesthrough a center of gravity of the area of the first optical reflectivelayer.

REFERENCE SIGNS LIST

-   11 substrate (GaN substrate)-   20 laminate structural body-   21 n-type compound semiconductor layer-   21A n-contact layer-   21B n-clad layer-   21 a first surface of n-type compound semiconductor layer-   21 b second surface of n-type compound semiconductor layer-   21 c convex portion provided on n-type compound semiconductor layer-   21 d side surface (side wall) of convex portion-   21 e concave portion surrounding convex portion-   22 p-type compound semiconductor layer-   22A electron barrier layer-   22B p-clad layer-   22C p-contact layer-   22 a first surface of p-type compound semiconductor layer-   22 b second surface of p-type compound semiconductor layer-   23 active layer-   24 insulating layer-   25 n-side electrode-   26 p-side electrode-   27 ridge stripe structure-   28 dielectric layer-   29 insulating material layer-   31, 31 ₁, 31 ₂ well layers-   32 barrier layer-   33, 31 ₁, 31 ₂ tunnel barrier layers-   41 n-side electrode-   42 p-side electrode-   43 current confinement layer-   43A opening of current confinement layer-   44 pad electrode-   45 bonding layer-   46 support substrate-   51 first optical reflective layer-   52 second optical reflective layer-   53 expansion mitigation film-   54 dielectric film

The invention claimed is:
 1. An optical semiconductor device,comprising: a laminate structural body that comprises: an n-typecompound semiconductor layer; an active layer; a p-type compoundsemiconductor layer, wherein an order of arrangement is the n-typecompound semiconductor layer, the active layer, and the p-type compoundsemiconductor layer, respectively, the active layer includes amultiquantum well structure including a first well layer, a first tunnelbarrier layer, a first barrier layer, a second tunnel barrier layer, anda second well layer, the first barrier layer is between the first tunnelbarrier layer and the second tunnel barrier layer, and a compositionalvariation of the first well layer is greater than a compositionalvariation of the second well layer; an electron barrier layer adjacentto the active layer; and a non-doped compound semiconductor layerbetween the active layer and the electron barrier layer.
 2. The opticalsemiconductor device according to claim 1, wherein the non-dopedcompound semiconductor layer is a non-doped InGaN layer.
 3. The opticalsemiconductor device according to claim 1, wherein the non-dopedcompound semiconductor layer is a non-doped AlGaN layer.
 4. The opticalsemiconductor device according to claim 1, wherein the electron barrierlayer is a p-type AlGaN.